Graphemes: add fast-path for printable-ASCII

[charlievieth]

Speedup the handling of strings that contain only printable-ASCII characters by ~20x.

TODO: this commit should probably handle common, but "non-printable" chars like: "\n", "\r".

base: d6773baf

goos: darwin
goarch: arm64
pkg: github.com/rivo/uniseg

name                    old time/op    new time/op    delta
GraphemesNext-10          2.73µs ± 0%    2.74µs ± 0%   +0.31%  (p=0.008 n=5+5)
GraphemesNext_ASCII-10    3.82µs ± 0%    0.19µs ± 1%  -95.10%  (p=0.008 n=5+5)

name                    old alloc/op   new alloc/op   delta
GraphemesNext-10           0.00B          0.00B          ~     (all equal)
GraphemesNext_ASCII-10     0.00B          0.00B          ~     (all equal)

name                    old allocs/op  new allocs/op  delta
GraphemesNext-10            0.00           0.00          ~     (all equal)
GraphemesNext_ASCII-10      0.00           0.00          ~     (all equal)

base: charlievieth/cev/perf

name                    old time/op    new time/op    delta
GraphemesNext-10           466ns ± 0%     501ns ± 0%   +7.47%  (p=0.008 n=5+5)
GraphemesNext_ASCII-10     827ns ± 0%     188ns ± 1%  -77.28%  (p=0.008 n=5+5)

name                    old alloc/op   new alloc/op   delta
GraphemesNext-10           0.00B          0.00B          ~     (all equal)
GraphemesNext_ASCII-10     0.00B          0.00B          ~     (all equal)

name                    old allocs/op  new allocs/op  delta
GraphemesNext-10            0.00           0.00          ~     (all equal)
GraphemesNext_ASCII-10      0.00           0.00          ~     (all equal)

[rivo]

I'm wondering why you would use this package if you only deal with printable ASCII characters.

[dchapes]

@charlievieth again, as with my previous comment, if this package uses an internal string representation it should just ditch the []rune conversion entirely.

[rivo]

I prefer not to add these kind of exceptions.