gccrs: Add Parser for Rust front-end pt.1

gccrs: Add Parser for Rust front-end pt.1

This is a Pratt-style parser for Rust that implements all of the AST. The
rust-parser-impl.h is the implementation of the parser as a template,
allowing it to be given ManagedTokenSource and avoid virtual calls. The
downside is it takes time to compile when used.

see: https://en.wikipedia.org/wiki/Operator-precedence_parser#Pratt_parsing

This patch contains the first half of the templated parser, so as to not
lose patches in the mailing list archives.

	gcc/rust/
	* parse/rust-cfg-parser.cc: New.
	* parse/rust-cfg-parser.h: New.
	* parse/rust-parse-impl.h: New.
	* parse/rust-parse.cc: New.
	* parse/rust-parse.h: New.

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