text/template: replace bytes.Buffer with strings.Builder

After Go.1.10+ strings.Builder is known as more efficient in
concatenating and building strings than bytes.Buffer.

In this CL,
there is a minor logic fix for getting advantage of strings.builder.

name               old time/op    new time/op    delta
DefinedTemplate-8     543ns ± 3%     512ns ± 2%   -5.73%  (p=0.000 n=8+8)

name               old alloc/op   new alloc/op   delta
DefinedTemplate-8      192B ± 0%      160B ± 0%  -16.67%  (p=0.000 n=8+8)

name               old allocs/op  new allocs/op  delta
DefinedTemplate-8      5.00 ± 0%      5.00 ± 0%     ~     (all equal)

Change-Id: Icda0054d146e6c5e32ed8a4d13221bb6850d31b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/175261


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