text/template/parse: use strings.Builder in Chain and List nodes

This CL is a continuation of 198078. Benchmark output:

benchmark                     old ns/op     new ns/op     delta
BenchmarkParseLarge-8         24759165      24516563      -0.98%
BenchmarkVariableString-8     115           115           +0.00%
BenchmarkListString-8         924           680           -26.41%

benchmark                     old allocs     new allocs     delta
BenchmarkVariableString-8     3              3              +0.00%
BenchmarkListString-8         14             13             -7.14%

benchmark                     old bytes     new bytes     delta
BenchmarkVariableString-8     72            72            +0.00%
BenchmarkListString-8         512           424           -17.19%

Change-Id: I9ec48fe4832437c556a5fa94d4cbf6e29e28d944
Reviewed-on: https://go-review.googlesource.com/c/go/+/198080


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>