perf: optimize identifier parsing

[DaniPopes]

Motivation

We know that all identifiers in Solidity are ASCII, so we don't need to bother with multi-byte characters. As a bonus, we also get to make is_valid_identifier const (!).

Note that this was initially just going to replace s.chars() with s.as_bytes().iter().map(|x| *x as char), but we can go a step further and remove iterators to make the function const.

Godbolt

I also split the parsing benchmark into parsing and resolving, so below is the before/after.

resolve bench should be mostly noise since that is not touched.

Before:

parse/keywords          time:   [29.776 ns 29.803 ns 29.830 ns]                            
parse/complex           time:   [721.65 ns 722.08 ns 722.58 ns]                           

resolve/keywords        time:   [28.775 ns 28.798 ns 28.824 ns]                              
resolve/complex         time:   [658.11 ns 658.53 ns 658.98 ns]                             

After:

parse/keywords          time:   [27.398 ns 27.425 ns 27.453 ns]                            
                        change: [-9.6446% -9.1972% -8.8178%] (p = 0.00 < 0.05)
                        Performance has improved.
parse/complex           time:   [583.73 ns 584.82 ns 586.14 ns]                           
                        change: [-17.511% -17.085% -16.524%] (p = 0.00 < 0.05)
                        Performance has improved.

resolve/keywords        time:   [29.062 ns 29.166 ns 29.322 ns]                              
                        change: [+2.0503% +2.4201% +2.8360%] (p = 0.00 < 0.05)
                        Change within noise threshold.
resolve/complex         time:   [716.83 ns 718.96 ns 721.46 ns]                             
                        change: [+6.8756% +7.2941% +7.6725%] (p = 0.00 < 0.05)
                        Performance has regressed.

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes