mime: speed up ParseMediaType

[jub0bs]

Add benchmarks for ParseMediaType.

Eschew UTF-8 decoding and strings.IndexFunc where possible, and rely
on 128-bit bitmaps instead. Eliminate some bounds checks.

Some benchmark results (no changes to allocations):

goos: darwin
goarch: amd64
pkg: mime
cpu: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
│ old │ new │
│ sec/op │ sec/op vs base │
ParseMediaType-8 71.75µ ± 0% 55.53µ ± 0% -22.60% (p=0.000 n=20)
ParseMediaTypeBogus-8 5.330µ ± 0% 3.603µ ± 0% -32.41% (p=0.000 n=20)
geomean 19.56µ 14.14µ -27.67%

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