time/format: avoid growslice in time.String()/time.GoString()

Pre-allocate the slice of buf with enough capacity
to avoid growslice calls.

benchmark                   old ns/op     new ns/op     delta
BenchmarkTimeString-4       493           409           -17.12%
BenchmarkTimeGoString-4     309           182           -41.30%

benchmark                   old allocs     new allocs     delta
BenchmarkTimeString-4       5              3              -40.00%
BenchmarkTimeGoString-4     4              1              -75.00%

benchmark                   old bytes     new bytes     delta
BenchmarkTimeString-4       152           128           -15.79%
BenchmarkTimeGoString-4     248           80            -67.74%

Change-Id: I64eabe2ab0b3d4a846453c2e8e548a831d720b8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/343971


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Alexander Rakoczy <alex@golang.org>