time: add a fast path for the IMF-fixdate layout in AppendFormat

[davidteather]

AppendFormat special-cases RFC3339 and RFC3339Nano, every other layout
falls through to the general formatter, which re-parses the layout on
each call. IMF-fixdate is the format RFC 9110 requires HTTP senders to
generate, so it is re-parsed once per response by every call site that
writes a date header.

Add a specialization for it. GMT is a literal in this layout rather
than a zone abbreviation, so the time is formatted in its own location
exactly as the general path formats it, supplying UTC remains the
caller's responsibility.

The switch dispatches on layout length before comparing contents, so
RFC3339 is unaffected. Layouts sharing this one's length pay one
additional comparison. Both are covered by the benchmarks below.

goos: darwin
goarch: arm64
pkg: time
cpu: Apple M2 Pro
│ old │ new │
│ sec/op │ sec/op vs base │
Format-10 169.4n ± 0% 169.5n ± 0% ~ (p=0.697 n=25)
FormatRFC3339-10 107.9n ± 0% 107.8n ± 0% ~ (p=0.821 n=25)
FormatRFC3339Nano-10 109.9n ± 0% 109.6n ± 0% ~ (p=0.273 n=25)
FormatIMFFixdate-10 179.0n ± 1% 104.6n ± 0% -41.56% (p=0.000 n=25)
FormatRFC1123-10 179.8n ± 0% 180.0n ± 0% ~ (p=0.534 n=25)
FormatNow-10 105.1n ± 0% 105.8n ± 1% +0.67% (p=0.000 n=25)
geomean 137.6n 125.9n -8.49%

B/op and allocs/op are unchanged.

No caller needs to change. In the standard library this layout is used
by net/http, net/http/internal/http2, net/http/fcgi and net/http's file
server. net/http's Date header moved from a hand-rolled formatter to
AppendFormat in CL 647955. This makes that simplification free.

Fixes #80751

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from David Teather:

Patch Set 1:

(1 comment)

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