Remove SkSL::String class.

The previous CLs have removed the last significant differences between
SkSL::String and std::string. This CL removes SkSL::String entirely and
replaces it with std::string throughout the code.

Apologies for the very long CL, but I have done my best to make it as
simple and reviewable as possible. The vast majority of changes are
simple replacement of `SkSL::String` with `std::string`. In the rare
spots where code is moved from one place to another, it is logically
unchanged.

Change-Id: I39563d2db45da229f17f4504dfd63e00bde7a96e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503339
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
171 files changed