Add SkMakeSpan and helpful conversion

SkMakeSpan uses function type inference to remove boilerplate
code. The converting casts simplifies dealing with T* to const T*
uses.

Change-Id: I1851e144c4e530c275710514ce30ad75a7eb94c6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/225192
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
10 files changed