Move SkSpan to include/, for use in public API

Change-Id: I674f038600afd6d49316c1ece515941ee5579068
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/406939
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
37 files changed