gccrs: ast: Add take_items() and set_items() methods for Item containers

gccrs: ast: Add take_items() and set_items() methods for Item containers

Both the AST::Crate and AST::Module class are std::unique_ptr<AST::Item>
containers, and may require spurious insertions in these containers,
for example when expanding a procedural macro, or in our case, escaping
macros through the #[macro_use] attribute. These functions allow you
to replace *all* of the items contained in such a container.

gcc/rust/ChangeLog:

	* ast/rust-ast.h: Add take_items() and set_items() method to Crate.
	* ast/rust-item.h: Add take_items() and set_items() method to Module.