SkSL DSL operator overloading

This adds basic operator overloading (everything but array indexing) and
related tests to the SkSL DSL.

Change-Id: Ic9fdc9a02a5496e2706d18fb435d838b4ee53ad7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/353103
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
9 files changed