TOML: new PEG based parser

[masatake]

The peg file is imported from

Copyright (c) Tom Preston-Werner
https://github.com/toml-lang/toml/blob/main/toml.abnf

• a subparser of Cargo.toml (https://doc.rust-lang.org/cargo/index.html)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.49%. Comparing base (a9e213c) to head (aed1e7f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3509      +/-   ##
==========================================
+ Coverage   85.45%   85.49%   +0.03%     
==========================================
  Files         235      236       +1     
  Lines       56848    56970     +122     
==========================================
+ Hits        48582    48705     +123     
+ Misses       8266     8265       -1     

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[masatake]

It seems that a bug in pegof.

[dolik-rce]

First problem I found is that pegof doesn't currently support unicode escapes in strings, e.g. underscore <- '\u005F'. It should be easy to implement.

[masatake]

@dolik-rce Thank you.