Parser/lexer v8 optimizations

[DimitrisJim]

Reference: https://v8.dev/blog/scanner and https://v8.dev/blog/preparser (and probably others in the blog page)

Has some nice ideas on optimizations for the scanning and parsing phases.

[DimitrisJim]

Identifier scanning

We can also do a very simple variant of this, instead of using a map we can just match on the ascii characters instead, basically:

    fn is_identifier_start(&self, c: char) -> bool {
        match c {
            'a'..='z' | 'A'..='Z' | '_' =>  true,
            c if is_xid_start(c) =>  true,
            _ => false
        }
    }

    fn is_identifier_continuation(&self) -> bool {
        match self.window[0] {
            Some('a'..='z' | 'A'..='Z' | '_' | '0'..='9') => true,
            Some(c) => is_xid_continue(c),
            _ => false,
        }
    }

i.e, adding the 'a'..='z' and 'A'..='Z' cases.

Timings

Definitely an improvement here:

lexer                   time:   [119.18 µs 119.26 µs 119.34 µs]                  
                        change: [-16.065% -14.664% -13.205%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)