New Rust lexer + ctags generator

This is a near complete rewrite of the work in PR #165. Additionally, it brings in the Rust lexer from Scintilla 3.3.6.

The ctags generator generates tags for structs, functions, traits, modules, macros, enums, implementations and static variables/constants. Currently it maps them to analogous items from the C language family, although a commented out block in src/symbols.c has the proper item names for Rust (it is commented out as I'm not sure how to deal with internationalization/whether this is worth doing).

Note that since this is a near-complete rewrite, many of the initial commits are not very relevant... I can squash them if that is preferred.