runtime: special-case small byte appends.
Update issue 3679.

BenchmarkAppend1Byte            484          199  -58.88%
BenchmarkAppend4Bytes           829          286  -65.50%
BenchmarkAppend8Bytes           484          365  -24.59%
BenchmarkAppend16Bytes          484          498   +2.89%
BenchmarkAppend32Bytes          486          484   -0.41%

R=iant, dave, rsc
CC=golang-dev
https://codereview.appspot.com/7443047