go/printer: reduce allocations to improve performance

First, we know that Go source files almost always weigh at least a few
kilobytes, so we can kickstart the output buffer to be a reasonable size
and reduce the initial number of incremental allocations and copies when
appending bytes or strings to output.

Second, in nodeSize we use a nested printer, but we don't actually need
its printed bytes - we only need to know how many bytes it prints.
For that reason, use a throwaway buffer: the part of our output buffer
between length and capacity, as we haven't used it yet.

Third, use a sync.Pool to reuse allocated printers.
The current API doesn't allow reusing printers,
and some programs like gofmt will print many files in sequence.

Those changes combined result in a modest reduction in allocations and
CPU usage. The benchmark uses testdata/parser.go, which has just over
two thousand lines of code, which is pretty standard size-wise.

We also split the Print benchmark to cover both a medium-sized ast.File
as well as a pretty small ast.Decl node. The latter is a somewhat common
scenario in gopls, which has code actions which alter small bits of the
AST and print them back out to rewrite only a few lines in a file.

	name          old time/op    new time/op     delta
	PrintFile-16    5.43ms ± 1%     4.85ms ± 3%  -10.68%  (p=0.000 n=9+10)
	PrintDecl-16    19.1µs ± 0%     18.5µs ± 1%   -3.04%  (p=0.000 n=10+10)

	name          old speed      new speed       delta
	PrintFile-16  9.56MB/s ± 1%  10.69MB/s ± 3%  +11.81%  (p=0.000 n=8+10)
	PrintDecl-16  1.67MB/s ± 0%   1.73MB/s ± 1%   +3.05%  (p=0.000 n=10+10)

	name          old alloc/op   new alloc/op    delta
	PrintFile-16     332kB ± 0%      107kB ± 2%  -67.87%  (p=0.000 n=10+10)
	PrintDecl-16    3.92kB ± 0%     3.28kB ± 0%  -16.38%  (p=0.000 n=10+10)

	name          old allocs/op  new allocs/op   delta
	PrintFile-16     3.45k ± 0%      2.42k ± 0%  -29.90%  (p=0.000 n=10+10)
	PrintDecl-16      56.0 ± 0%       46.0 ± 0%  -17.86%  (p=0.000 n=10+10)

Change-Id: I475a3babca77532b2d51888f49710f74763d81d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/424924
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

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