The go:nosplit change wasn't the problem, reinstating.

««« original CL description
undo CL 93380044 / 7f0999348917

Partial undo, just of go:nosplit annotation.  Somehow it
is breaking the windows builders.

TBR=bradfitz

««« original CL description
runtime: implement string ops in Go

Also implement go:nosplit annotation.  Not really needed
for now, but we'll definitely need it for other conversions.

benchmark                 old ns/op     new ns/op     delta
BenchmarkRuneIterate      534           474           -11.24%
BenchmarkRuneIterate2     535           470           -12.15%

LGTM=bradfitz
R=golang-codereviews, dave, bradfitz, minux
CC=golang-codereviews
https://golang.org/cl/93380044
»»»

TBR=bradfitz
CC=golang-codereviews
https://golang.org/cl/105260044
»»»

TBR=bradfitz
R=bradfitz, golang-codereviews
CC=golang-codereviews
https://golang.org/cl/103490043