Use map[string]struct{} for compression map in Len

[tmthrgd]

map[string]int requires 8 bytes per entry to store the unused position information.

I couldn't get any clean benchmarks from this, but map[string]struct{} definitely uses less memory than map[string]int.

This does unfortunately make the related test cases slightly weaker as they no longer test that the Len code is finding compression points in the correct spot.

Updates #709.

Edit: I was able to get some better benchmarks that show a very notable improvement in memory use.

$ benchstat {old,new}.bench
name                       old time/op    new time/op    delta
MsgLength-12                  329ns ± 0%     328ns ± 0%   -0.27%  (p=0.040 n=9+9)
MsgLengthNoCompression-12    9.37ns ± 1%    9.34ns ± 2%     ~     (p=0.099 n=10+10)
MsgLengthPack-12             2.26µs ± 9%    2.36µs ±23%     ~     (p=0.780 n=9+10)
MsgLengthMassive-12          63.1µs ± 8%    53.0µs ± 3%  -15.89%  (p=0.000 n=10+10)

name                       old alloc/op   new alloc/op   delta
MsgLength-12                  32.0B ± 0%     32.0B ± 0%     ~     (all equal)
MsgLengthNoCompression-12     0.00B          0.00B          ~     (all equal)
MsgLengthPack-12               896B ± 0%      896B ± 0%     ~     (all equal)
MsgLengthMassive-12          20.2kB ± 0%    14.8kB ± 0%  -26.74%  (p=0.000 n=8+9)

name                       old allocs/op  new allocs/op  delta
MsgLength-12                   1.00 ± 0%      1.00 ± 0%     ~     (all equal)
MsgLengthNoCompression-12      0.00           0.00          ~     (all equal)
MsgLengthPack-12               8.00 ± 0%      8.00 ± 0%     ~     (all equal)
MsgLengthMassive-12             138 ± 0%       138 ± 0%     ~     (all equal)

[codecov-io]

Codecov Report

Merging #820 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #820      +/-   ##
==========================================
- Coverage   57.88%   57.84%   -0.05%     
==========================================
  Files          42       42              
  Lines       10824    10824              
==========================================
- Hits         6266     6261       -5     
- Misses       3473     3477       +4     
- Partials     1085     1086       +1

Impacted Files	Coverage Δ
msg.go	77.93% <100%> (-0.65%)	⬇️
zcompress.go	26.24% <100%> (ø)	⬆️
server.go	68.44% <0%> (-0.25%)	⬇️

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[miekg]

I would have sworn this length was somehow needed, guess not.

[miekg]

sorry, went to the wrong way through the PRs, created a merge conflict /o\

[miekg]

lgtm

[tmthrgd]

These were going to create a merge conflict regardless of ordering. I've manually rebased.