Improved DSL APIs in preparation for DSLParser

This includes several new array variants of existing APIs, DSLBlock now
supporting SymbolTables, and a couple of other minor changes needed by
the upcoming DSLParser.

Change-Id: I71feb268feb27cf7ff453cc59046091779bffe06
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/404779
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
14 files changed