Remove custom iterators from SkSL::Program

Instead, just expose the vector directly (as elements()).

Change-Id: I9f6a3ae38cd8e6f1b0a1087a42d61452fe883924
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324130
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
13 files changed