strings: add Clone function

strings: add Clone function

The new strings.Clone function copies the input string
without the returned cloned string referencing the
input strings memory.

goarch: amd64
cpu: Intel(R) Core(TM) i5-1038NG7 CPU @ 2.00GHz

name     time/op
Clone-8  24.2ns ± 2%

name     alloc/op
Clone-8   48.0B ± 0%

name     allocs/op
Clone-8    1.00 ± 0%

Update #45038
Fixes #40200

Change-Id: Id9116c21c14328ec3931ef9a67a2e4f30ff301f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/345849
Trust: Martin Möhrmann <martin@golang.org>
Run-TryBot: Martin Möhrmann <martin@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>