text/template/parse: use strings.Builder in String methods
As mentioned in godoc, strings.Builder is more efficient for
concatenating and building strings.
Running a simple bench test on VariableNode.String() gives:
benchmark old ns/op new ns/op delta
BenchmarkParseLarge-8
25676831 24453285 -4.77%
BenchmarkVariableString-8 296 115 -61.15%
benchmark old allocs new allocs delta
BenchmarkVariableString-8 8 3 -62.50%
benchmark old bytes new bytes delta
BenchmarkVariableString-8 112 72 -35.71%
Change-Id: I13c9340080738fcad1edeed859d33ba608e4b05a
Reviewed-on: https://go-review.googlesource.com/c/go/+/198078
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>