gccrs: format-args: Start storing string in Rust memory

gcc/rust/ChangeLog:

	* ast/rust-fmt.cc (ffi::RustHamster::to_string): New.
	(Pieces::collect): Adapt to use new handle API.
	(Pieces::~Pieces): Likewise.
	(Pieces::Pieces): Likewise.
	(Pieces::operator=): Likewise.
	* ast/rust-fmt.h (struct RustString): Add members.
	(struct FormatArgsHandle): New.
	(clone_pieces): Adapt for new FFI API.
	(destroy_pieces): Likewise.
	(struct Pieces): Store new FormatArgsHandle type.
	* expand/rust-expand-format-args.cc (expand_format_args): Use proper
	namespace.
	* resolve/rust-ast-resolve-base.cc (ResolverBase::visit): FormatArgs
	nodes are already resolved, so do nothing.

libgrust/ChangeLog:

	* libformat_parser/src/lib.rs: Use new Handle struct and expose it.