gccrs: Add Rust front-end base AST data structures

gccrs: Add Rust front-end base AST data structures

This is a full C++11 class hierarchy representing the Rust AST. We do not
allow dynamic_cast and so the main mechanism to work with the AST is by
using the visitor interface. Slowly we are adding TREE_CODE style node
types to the AST which will allow for more ways to work with the AST but
for now this is it.

See: https://doc.rust-lang.org/reference/items.html

	gcc/rust/
	* ast/rust-ast-full-decls.h: New.
	* ast/rust-ast-full-test.cc: New.
	* ast/rust-ast-full.h: New.
	* ast/rust-ast.h: New.
	* operator.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>