Pull SkSpan into its own header file

Change-Id: I32d800c91794093973cb8128fd1b83b4aff91845
Reviewed-on: https://skia-review.googlesource.com/148672
Commit-Queue: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
3 files changed