bufio: optimize bufio.Reader.ReadString to avoid an allocation and copy

name                old time/op    new time/op    delta
ReaderReadString-4     226ns ±12%     161ns ±11%  -28.76%  (p=0.008 n=5+5)

name                old alloc/op   new alloc/op   delta
ReaderReadString-4      288B ± 0%      144B ± 0%  -50.00%  (p=0.008 n=5+5)

name                old allocs/op  new allocs/op  delta
ReaderReadString-4      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)

Change-Id: I77f330b8340c2bfbfff1f6f1000170b65953a200
GitHub-Last-Rev: 65d65302a7b80504b4d37b81a3843fe1439e638a
GitHub-Pull-Request: golang/go#34706
Reviewed-on: https://go-review.googlesource.com/c/go/+/199257


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>