perf: speed up v8::String::to_rust_*_lossy()

[lucacasonato]

This commit speeds up this common conversion
method between by 2x for many common cases. Short
one byte ASCII strings are now 20% faster. Longer
one byte ASCII strings are 2.5x faster. Short UTF8
strings are marginally slower (5%) but longer UTF8
strings are upwards of 2x faster.

A follow up will make the short UTF8 strings about
2x faster than the current implementation as well.

[lucacasonato]

Main

ascii alloc: 28.693481ms
asciilong alloc: 12.713113ms
utf8 alloc: 77.509234ms
utf8long alloc: 415.201443ms
ascii stack: 19.645841ms
asciilong stack: 11.865463ms
utf8 stack: 66.01316ms
utf8long stack: 421.637861ms

ascii alloc: 28.82945ms
asciilong alloc: 12.715254ms
utf8 alloc: 77.888193ms
utf8long alloc: 424.717064ms
ascii stack: 19.456001ms
asciilong stack: 11.794382ms
utf8 stack: 64.327569ms
utf8long stack: 420.072509ms

ascii alloc: 27.680299ms
asciilong alloc: 12.362603ms
utf8 alloc: 77.129383ms
utf8long alloc: 424.482294ms
ascii stack: 20.359151ms
asciilong stack: 12.390033ms
utf8 stack: 66.729982ms
utf8long stack: 422.581681ms

This PR

ascii alloc: 24.014995ms
asciilong alloc: 5.197276ms
utf8 alloc: 80.071666ms
utf8long alloc: 267.608056ms
ascii stack: 16.353647ms
asciilong stack: 5.159856ms
utf8 stack: 71.056516ms
utf8long stack: 273.113161ms

ascii alloc: 23.978715ms
asciilong alloc: 5.238166ms
utf8 alloc: 79.217804ms
utf8long alloc: 265.617734ms
ascii stack: 15.503417ms
asciilong stack: 5.143925ms
utf8 stack: 70.102775ms
utf8long stack: 264.238623ms

ascii alloc: 24.224156ms
asciilong alloc: 5.121675ms
utf8 alloc: 81.940328ms
utf8long alloc: 268.783537ms
ascii stack: 15.651557ms
asciilong stack: 5.150135ms
utf8 stack: 72.978149ms
utf8long stack: 266.422574ms