When growing slice take into account size of the allocated memory block.
Also apply the same optimization to string->[]byte conversion.
Fixes #6307.

benchmark                    old ns/op    new ns/op    delta
BenchmarkAppendGrowByte        4541036      4434108   -2.35%
BenchmarkAppendGrowString     59885673     44813604  -25.17%

LGTM=khr
R=khr
CC=golang-codereviews, iant, rsc
https://golang.org/cl/53340044