Improve performance of Path#dirname and Path#extension

[BlobCodes]

This PR improves the performance of the Path#dirname and Path#extension methods.
The new methods are also smaller.

#dirname Benchmarks

Benchmark: Path["😎"].dirname
old  56.18M ( 17.80ns) (± 1.34%)  0.0B/op   1.30× slower
new  73.21M ( 13.66ns) (± 0.75%)  0.0B/op        fastest
===
Benchmark: Path["C://"].dirname
old  28.56M ( 35.01ns) (± 0.16%)  0.0B/op   2.44× slower
new  69.58M ( 14.37ns) (± 0.44%)  0.0B/op        fastest
===
Benchmark: Path["C://Users/"].dirname
old  12.08M ( 82.77ns) (± 0.53%)  16.0B/op   2.39× slower
new  28.90M ( 34.60ns) (± 1.97%)  16.0B/op        fastest
===
Benchmark: Path["i/🇱 🇮 🇰 🇪/😎.uni🇨ode"].dirname
old   8.12M (123.19ns) (± 0.83%)  48.0B/op   1.44× slower
new  11.71M ( 85.39ns) (± 1.40%)  48.0B/op        fastest
===
Benchmark: Path["trailing/sepe.rators///"].dirname
old   7.24M (138.13ns) (± 1.02%)  32.0B/op   2.65× slower
new  19.16M ( 52.20ns) (± 1.92%)  32.0B/op        fastest
===
Benchmark: Path["/hidden/.file"].dirname
old  13.62M ( 73.43ns) (± 1.11%)  32.0B/op   2.00× slower
new  27.25M ( 36.70ns) (± 3.91%)  32.0B/op        fastest
===
Benchmark: Path["/mnt"].dirname
old  17.63M ( 56.72ns) (± 1.07%)  48.0B/op   2.40× slower
new  42.32M ( 23.63ns) (± 1.93%)  16.0B/op        fastest
===
Benchmark: Path["."].dirname
old  75.72M ( 13.21ns) (± 0.92%)  0.0B/op   1.27× slower
new  95.96M ( 10.42ns) (± 0.19%)  0.0B/op        fastest
===
Benchmark: Path["file"].dirname
old  29.21M ( 34.23ns) (± 0.36%)  0.0B/op   2.52× slower
new  73.73M ( 13.56ns) (± 1.22%)  0.0B/op        fastest
===
Benchmark: Path["very/long/path_with_many/different_parts_to/test_the_performance_of_the_methods"].dirname
old   3.14M (318.56ns) (± 1.21%)  64.0B/op   2.61× slower
new   8.19M (122.08ns) (± 3.27%)  64.0B/op        fastest
===


Windows:
Benchmark: Path.windows("😎").dirname
old  52.28M ( 19.13ns) (± 0.71%)  0.0B/op   1.15× slower
new  60.33M ( 16.58ns) (± 1.11%)  0.0B/op        fastest
===
Benchmark: Path.windows("C://").dirname
old  11.57M ( 86.46ns) (± 1.76%)  64.0B/op   1.26× slower
new  14.60M ( 68.47ns) (± 1.59%)  64.0B/op        fastest
===
Benchmark: Path.windows("C://Users/").dirname
old   7.70M (129.83ns) (± 2.36%)  64.0B/op        fastest
new   7.70M (129.85ns) (± 4.81%)   128B/op   1.00× slower
===
Benchmark: Path.windows("i/🇱 🇮 🇰 🇪/😎.uni🇨ode").dirname
old   7.94M (125.92ns) (± 2.09%)  48.0B/op   1.93× slower
new  15.30M ( 65.38ns) (± 1.56%)  48.0B/op        fastest
===
Benchmark: Path.windows("trailing/sepe.rators///").dirname
old   7.03M (142.30ns) (± 1.61%)  32.0B/op   2.60× slower
new  18.24M ( 54.83ns) (± 2.03%)  32.0B/op        fastest
===
Benchmark: Path.windows("/hidden/.file").dirname
old  13.31M ( 75.13ns) (± 0.97%)  32.0B/op   1.93× slower
new  25.69M ( 38.92ns) (± 2.83%)  32.0B/op        fastest
===
Benchmark: Path.windows("/mnt").dirname
old  17.35M ( 57.65ns) (± 0.74%)  48.0B/op   2.22× slower
new  38.44M ( 26.02ns) (± 2.35%)  16.0B/op        fastest
===
Benchmark: Path.windows(".").dirname
old  71.74M ( 13.94ns) (± 0.41%)  0.0B/op   1.07× slower
new  77.12M ( 12.97ns) (± 0.69%)  0.0B/op        fastest
===
Benchmark: Path.windows("file").dirname
old  27.96M ( 35.76ns) (± 0.05%)  0.0B/op   2.16× slower
new  60.28M ( 16.59ns) (± 0.63%)  0.0B/op        fastest
===
Benchmark: Path.windows("very/long/path_with_many/different_parts_to/test_the_performance_of_the_methods").dirname
old   3.06M (327.05ns) (± 1.38%)  64.0B/op   2.68× slower
new   8.21M (121.86ns) (± 1.22%)  64.0B/op        fastest
===

#extension Benchmarks

Benchmark: Path["test.ext/foo"].extension
old  57.58M ( 17.37ns) (± 2.22%)  0.0B/op   1.06× slower
new  61.14M ( 16.36ns) (± 0.53%)  0.0B/op        fastest
===
Benchmark: Path["my_test_dir/image.jpeg"].extension
old  22.38M ( 44.69ns) (± 1.20%)  32.0B/op   1.10× slower
new  24.72M ( 40.46ns) (± 1.18%)  32.0B/op        fastest
===
Benchmark: Path["my_test_dir//image.jpeg//"].extension
old  20.84M ( 47.99ns) (± 1.47%)  32.0B/op   1.11× slower
new  23.12M ( 43.26ns) (± 1.16%)  32.0B/op        fastest
===
Benchmark: Path["test"].extension
old  82.85M ( 12.07ns) (± 0.86%)  0.0B/op   1.44× slower
new 119.28M (  8.38ns) (± 1.74%)  0.0B/op        fastest
===
Benchmark: Path["path_with/multiple...dots"].extension
old  21.50M ( 46.51ns) (± 0.77%)  32.0B/op   1.11× slower
new  23.93M ( 41.79ns) (± 1.23%)  32.0B/op        fastest
===
Benchmark: Path[".dotfile"].extension
old  51.82M ( 19.30ns) (± 1.92%)  0.0B/op   1.42× slower
new  73.41M ( 13.62ns) (± 0.91%)  0.0B/op        fastest
===
Benchmark: Path["path/to/.dotfile"].extension
old  41.21M ( 24.26ns) (± 3.96%)  0.0B/op   1.28× slower
new  52.60M ( 19.01ns) (± 2.82%)  0.0B/op        fastest
===
Benchmark: Path["path/archive.tar.gz"].extension
old  23.93M ( 41.78ns) (± 1.40%)  32.0B/op   1.10× slower
new  26.32M ( 37.99ns) (± 1.09%)  32.0B/op        fastest
===
Benchmark: Path["file/ending/with/a/dot."].extension
old  59.40M ( 16.84ns) (± 1.98%)  0.0B/op   1.00× slower
new  59.68M ( 16.76ns) (± 1.00%)  0.0B/op        fastest
===
Benchmark: Path[""].extension
old 291.64M (  3.43ns) (± 0.07%)  0.0B/op   1.08× slower
new 314.09M (  3.18ns) (± 0.25%)  0.0B/op        fastest
===
Benchmark: Path["verylong/file.extensionnnnnnnnnnnnnnnnn"].extension
old  13.10M ( 76.33ns) (± 1.28%)  48.0B/op   1.30× slower
new  16.99M ( 58.85ns) (± 1.19%)  48.0B/op        fastest
===

All specs ran fine, also I tested the methods using the strings seen in the Benchmark.

[BlobCodes]

Hmm the #extension method seems to fail some specs (appearantly not all Path specs were in path_spec.cr.. hmm)
I'll probably revert it.

[straight-shoota]

This looks great, really.

To fix #extension I think you just need to make sure to find any separators after the last dot. bytes.rindex(offset: pos) { |byte| byte === '.' || byte.in?(sep) } should work for that. Then you can check if bytes.unsafe_fetch(dot_index) is actually . and return empty otherwise.

The specs are in file_spec because these methods were originally implemented as class methods of File. We just moved the implementation to Path when that was introduced, and I kept the original specs exactly like they were to demonstrate backwards-compatibility. I think we should better move them to path_spec, though (that should better be a separate PR).

Your code comments in #dirname are really great and explain what's going on really well. #extname on the other hand could use a couple more on the special cases handling. The previous ones could just carry over.

[BlobCodes]

It seems like Path#extension now properly works.
It is not as fast as before, but still an improvement.

[straight-shoota]

Can you please fix the spelling of occurrEnce and sepArator in the code comments?

[sdogruyol]

🚀 Thank you @BlobCodes 🙏