runtime: faster strings

runtime: faster strings
Use FlagNoPointers and do not zeroize memory when allocate strings.
test/garbage/parser.out        old         new
run #1                     32.923s     32.065s
run #2                     33.047s     31.931s
run #3                     32.702s     31.841s
run #4                     32.718s     31.838s
run #5                     32.702s     31.868s

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5259041