Switch to the new SkSL lexer.

This completely replaces flex with a new in-house lexical analyzer generator,
which we have done for performance and memory usage reasons. Flex requires us
to copy strings every time we need the text of a token, whereas this new lexer
allows us to handle strings as a (non-null-terminated) pointer and length
everywhere, eliminating most string copies.

Bug: skia:
Change-Id: I2add26efc9e20cb699520e82abcf713af3968aca
Reviewed-on: https://skia-review.googlesource.com/39780
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
126 files changed