gccrs: libformat_parser: Send boxed values across FFI properly

gcc/rust/ChangeLog:

	* ast/rust-fmt.cc (Pieces::~Pieces): Call libformat_parser's release
	function in destructor.
	* ast/rust-fmt.h (struct PieceSlice): Add capacity.
	(destroy_pieces): New.
	(struct Pieces): Add destructor.

libgrust/ChangeLog:

	* libformat_parser/src/lib.rs: Leak Boxes properly for C++ to
	see them, add memory release function.