Add ASCII fast path from rust-lang

[epage]

See rust-lang/rust's src/librustc_lexer/src/lib.rs

Idea came from #13

[epage]

Used #21 to verify the performance impact

nicodeXID/is_xid_start/unicode
                        time:   [123.69 us 124.69 us 125.78 us]
                        thrpt:  [93.159 MiB/s 93.978 MiB/s 94.738 MiB/s]
                 change:
                        time:   [+13.170% +14.656% +16.131%] (p = 0.00 < 0.05)
                        thrpt:  [-13.890% -12.782% -11.638%]
                        Performance has regressed.
Found 13 outliers among 100 measurements (13.00%)
  4 (4.00%) high mild
  9 (9.00%) high severe
UnicodeXID/is_xid_start/ascii
                        time:   [273.80 ns 275.31 ns 276.97 ns]
                        thrpt:  [437.30 MiB/s 439.93 MiB/s 442.36 MiB/s]
                 change:
                        time:   [-75.168% -74.925% -74.661%] (p = 0.00 < 0.05)
                        thrpt:  [+294.66% +298.80% +302.70%]
                        Performance has improved.
Found 8 outliers among 100 measurements (8.00%)
  1 (1.00%) low mild
  6 (6.00%) high mild
  1 (1.00%) high severe
UnicodeXID/is_xid_continue/unicode
                        time:   [153.25 us 154.10 us 155.12 us]
                        thrpt:  [75.541 MiB/s 76.039 MiB/s 76.464 MiB/s]
                 change:
                        time:   [+44.472% +46.282% +47.760%] (p = 0.00 < 0.05)
                        thrpt:  [-32.323% -31.639% -30.782%]
                        Performance has regressed.
Found 11 outliers among 100 measurements (11.00%)
  1 (1.00%) low severe
  1 (1.00%) low mild
  1 (1.00%) high mild
  8 (8.00%) high severe
UnicodeXID/is_xid_continue/ascii
                        time:   [96.254 ns 97.036 ns 97.898 ns]
                        thrpt:  [1.2082 GiB/s 1.2189 GiB/s 1.2288 GiB/s]
                 change:
                        time:   [-91.110% -91.035% -90.961%] (p = 0.00 < 0.05)
                        thrpt:  [+1006.3% +1015.4% +1024.9%]
                        Performance has improved.

[SimonSapin]

If rustc contains a copy of this code, could efforts be unified?

Perhaps with a crate in a new repo in the rust-lang org, or even publishing to crates.io directly from rust-lang/rust?

[epage]

@SimonSapin if I'm reading librustc_lexer, it does pull in this crate. It just added a ascii fast path on top of it. Once librustc_lexer upgrades to a new release of this crate, the fast path will become redundant and could be removed.

[SimonSapin]

Ah I see, thanks!

[epage]

@Manishearth whats the release process for this crate? I'm wondering when I could take advantage of this PR.

[Manishearth]

@epage make a PR with a new version, I'll merge and publish

[epage]

@epage make a PR with a new version, I'll merge and publish

Sounds good, thanks!

#22