gccrs: Add name resolution pass to the Rust front-end

gccrs: Add name resolution pass to the Rust front-end

The name resolution is split into two phases, one toplevel pass which scans
the whole "Crate" which iterates all items and nested items in modules to
generate a context class full of CanonicalPath items. It also generates
a hierarchy of parent->child and child->parent relationships using the AST
NodeId for PathResolution in the second phase.

The second phase drills into each item like functions and creates a stack
of canonical paths for variables etc so that we can store information in
a side table of usage variable 'a' resolves to NodeId '123' which refers
to the NodeId of the "let a;" statement.

	gcc/rust/
	* resolve/rust-ast-resolve-base.cc: New.
	* resolve/rust-ast-resolve-base.h: New.
	* resolve/rust-ast-resolve-expr.cc: New.
	* resolve/rust-ast-resolve-expr.h: New.
	* resolve/rust-ast-resolve-implitem.h: New.
	* resolve/rust-ast-resolve-item.cc: New.
	* resolve/rust-ast-resolve-item.h: New.
	* resolve/rust-ast-resolve-path.cc: New.
	* resolve/rust-ast-resolve-path.h: New.
	* resolve/rust-ast-resolve-pattern.cc: New.
	* resolve/rust-ast-resolve-pattern.h: New.
	* resolve/rust-ast-resolve-stmt.cc: New.
	* resolve/rust-ast-resolve-stmt.h: New.
	* resolve/rust-ast-resolve-struct-expr-field.cc: New.
	* resolve/rust-ast-resolve-struct-expr-field.h: New.
	* resolve/rust-ast-resolve-toplevel.h: New.
	* resolve/rust-ast-resolve-type.cc: New.
	* resolve/rust-ast-resolve-type.h: New.
	* resolve/rust-ast-resolve.cc: New.
	* resolve/rust-ast-resolve.h: New.
	* resolve/rust-ast-verify-assignee.h: New.
	* resolve/rust-name-resolver.cc: New.
	* resolve/rust-name-resolver.h: New.