Fix #370 with better float EBNF and +inf -NaN +NaN parsing

[juntyr]

Fixes #370

Fixes the RON grammar for floats to be equivalent to FromStr's grammar (which we use internally):

float = ["+" | "-"], ("inf" | "NaN" | float_num);
float_num = (float_int | float_std | float_frac), [float_exp];
float_int = digit, { digit };
float_std = digit, { digit }, ".", {digit};
float_frac = ".", digit, {digit};
float_exp = ("e" | "E"), ["+" | "-"], digit, {digit};

Consequently, RON now also accepts +inf, -NaN, and +NaN.

In comparison with rustc, FromStr still accepts floats without a leading digit, e.g. .3 - these continue to work. The other way around, rustc accepts underscores in floats while FromStr and all other parsers I checked do not. I've added a special error FloatUnderscore to lint against this case.

• I've included my change in CHANGELOG.md

[codecov-commenter]

Codecov Report

Merging #371 (31a4b68) into master (46aa515) will increase coverage by 0.10%.
The diff coverage is 98.43%.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
+ Coverage   89.52%   89.63%   +0.10%     
==========================================
  Files          47       48       +1     
  Lines        5157     5219      +62     
==========================================
+ Hits         4617     4678      +61     
- Misses        540      541       +1     

Impacted Files	Coverage Δ
src/error.rs	7.89% <0.00%> (-0.11%)	⬇️
src/parse.rs	93.75% <100.00%> (+0.05%)	⬆️
tests/370_float_parsing.rs	100.00% <100.00%> (ø)

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[juntyr]

@kvark @torkleyy I think this should be a simple fix - unless you specifically did not want to accept +inf, -NaN, and +NaN. Explicitly checking for the underscores in floats is of course a bit extra, but might help if someone wanted to copy-paste their Rust struct directly into a RON config.

[kvark]

Love those tests!