This is the same technique used by Go's strings.Builder
(https://golang.org/src/strings/builder.go#L45), and for the same
reason. (We can't just use strings.Builder because there's no way to get
the underlying []byte to pass to usermem.IO.CopyIn.)
PiperOrigin-RevId: 240594892
Change-Id: Ic070e7e480aee53a71289c7c120850991358c52c