gccrs: Add Lexer for Rust front-end

gccrs: Add Lexer for Rust front-end

The lexer is referred to as a ManagedTokenSource within the parser. This
lexer does not currently support Unicode, but serves as a starting point
to do so.

	gcc/rust/
	* lex/rust-codepoint.h: New.
	* lex/rust-lex.cc: New.
	* lex/rust-lex.h: New.
	* lex/rust-token.cc: New.
	* lex/rust-token.h: New.
	* rust-buffered-queue.h: New.

Co-authored-by: Philip Herron <philip.herron@embecosm.com>
Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>
Co-authored-by: Mark Wielaard <mark@klomp.org>
Signed-off-by: Joel Phillips <simplytheother@gmail.com>