strings, internal/stringslite: lite version of strings package

To be used by internal/filepathlite, which is to be used by os.
There are probably other places where it would be convenient
to have strings functions accessible to RUNTIME level packages.

Change-Id: Icda59e7a9e26d9e8f3692db0ea4fb7b3dbf570d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/581516


Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>