Convert Operators namespace to first-class Operator object.

This lets us write `op.isAssignment()` instead of
`Operators::IsAssignment(op)`.

Change-Id: If35f2ac500b6ccabc364f9104faaad6e62564667
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369958
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
31 files changed