gccrs: Add declarations for Rust HIR

gccrs: Add declarations for Rust HIR

This patch contains the declarations needed for our second intermediate
representation, which we will refer to as an HIR.

This gives the front-end a chance to desugar much of the AST, such as:
- Removing distinction between functions and methods
- Removing Macros
- Removing IdentifierExprs
- Removing duplicate attribute structures

	gcc/rust/
	* hir/tree/rust-hir-expr.h: New.
	* hir/tree/rust-hir-item.h: New.
	* hir/tree/rust-hir-path.h: New.
	* hir/tree/rust-hir-pattern.h: New.
	* hir/tree/rust-hir-stmt.h: New.
	* hir/tree/rust-hir-type.h: New.