gccrs: rust-fmt: Store parsed string in Pieces struct

gcc/rust/ChangeLog:

	* ast/rust-fmt.cc (Pieces::collect): Fix signature to take ownership
	of the given string.
	* ast/rust-fmt.h (struct Pieces): Store parsed string in the struct.

libgrust/ChangeLog:

	* libformat_parser/src/lib.rs: Add debug prompt.