bufio: add fast path to WriteString for strings that fit in the buffer

[davidteather]

WriteString sets up its io.StringWriter fallback before checking
whether the string simply fits in the remaining buffer space, which
is the common case. The setup is not free: the interface value is
zeroed and state is spilled on every call, including the calls that
never use the fallback, making WriteString measurably slower than
Write for an identical payload that fits.

Handle the fits-in-buffer case first. The fallback loop then only
runs when the string does not fit, so its condition is known true on
entry: test it at the bottom of the loop instead so it is not
evaluated twice. The fallback path gets slightly faster as well.

goos: darwin
goarch: arm64
pkg: bufio
cpu: Apple M2 Pro
│ old │ new │
│ sec/op │ sec/op vs base │
WriterCopyOptimal-10 49.77n ± 0% 49.61n ± 1% -0.32% (p=0.017 n=25)
WriterCopyUnoptimal-10 49.98n ± 1% 49.82n ± 1% ~ (p=0.655 n=25)
WriterCopyNoReadFrom-10 2.295µ ± 2% 2.240µ ± 2% -2.40% (p=0.050 n=25)
WriterEmpty-10 470.2n ± 2% 463.3n ± 2% ~ (p=0.209 n=25)
WriterFlush-10 5.052n ± 1% 4.771n ± 1% -5.56% (p=0.000 n=25)
WriteString/small-10 4.774n ± 0% 3.529n ± 0% -26.08% (p=0.000 n=25)
WriteString/huge-10 5.757n ± 0% 5.469n ± 0% -5.00% (p=0.000 n=25)
geomean 44.98n 42.15n -6.31%

B/op and allocs/op are unchanged.

Fixes #80692

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Message from David Teather:

Patch Set 1:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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Message from Jorropo:

Patch Set 1: Code-Review+2 Commit-Queue+1

(4 comments)

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Patch Set 1:

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Patch Set 1: -Commit-Queue

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Message from David Teather:

Patch Set 2:

(4 comments)

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