mime: reduce allocs incurred by ParseMediaType

[jub0bs]

This change is mostly gardening. It simplifies ParseMediaType and its
helper functions and reduces the amount of allocations they incur.

Here are some benchmark results:

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 55.26µ ± 1% 54.54µ ± 1% -1.30% (p=0.000 n=20)
ParseMediaTypeBogus-8 3.551µ ± 0% 3.427µ ± 0% -3.48% (p=0.000 n=20)
geomean 14.01µ 13.67µ -2.39%

                  │     old      │                 new                 │
                  │     B/op     │     B/op      vs base               │

ParseMediaType-8 38.48Ki ± 0% 37.38Ki ± 0% -2.85% (p=0.000 n=20)
ParseMediaTypeBogus-8 2.531Ki ± 0% 2.469Ki ± 0% -2.47% (p=0.000 n=20)
geomean 9.869Ki 9.606Ki -2.66%

                  │    old     │                new                 │
                  │ allocs/op  │ allocs/op   vs base                │

ParseMediaType-8 457.0 ± 0% 425.0 ± 0% -7.00% (p=0.000 n=20)
ParseMediaTypeBogus-8 25.00 ± 0% 21.00 ± 0% -16.00% (p=0.000 n=20)
geomean 106.9 94.47 -11.62%

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from jub0bs:

Patch Set 1:

(1 comment)

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Message from Emmanuel Odeke:

Patch Set 2: Commit-Queue+1

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Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-09-21T20:37:59Z","revision":"2cd818d02d61de5d5c3d235c3c00dc3f81a4992c"}

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Patch Set 2: -Commit-Queue

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Patch Set 2:

This CL has passed the run

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Patch Set 2: LUCI-TryBot-Result+1

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Patch Set 2:

(2 comments)

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Patch Set 2:

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