faster string operations, and more

tested on linux/386

runtime_test.BenchmarkSliceToString                    642          532  -17.13%
runtime_test.BenchmarkStringToSlice                    636          528  -16.98%
runtime_test.BenchmarkConcatString                    1109          897  -19.12%

R=r, iant, rsc
CC=golang-dev
https://golang.org/cl/4674042