gccrs: Improve libformat_parser FFI

gccrs: Improve libformat_parser FFI

This should remove a use-after-free as well as simplify the FFI
interface.

gcc/rust/ChangeLog:

	* ast/rust-fmt.cc (Pieces::collect): Handle changes to ffi
	interface.
	(Pieces::~Pieces): Remove function definition.
	(Pieces::Pieces): Likewise.
	(Pieces::operator=): Likewise.
	* ast/rust-fmt.h: Include "optional.h".
	(rust_ffi_alloc): New extern "C" function declaration.
	(rust_ffi_dealloc): Likewise.
	(class FFIVec): New class.
	(class FFIOpt): Likewise.
	(RustHamster::RustHamster): New constructor accepting const
	std::string reference.
	(struct FormatSpec): Use FFIOpt.
	(struct PieceSlice): Remove struct.
	(struct RustString): Likewise.
	(struct FormatArgsHandle): Likewise.
	(collect_pieces): Change function signature.
	(clone_pieces): Likewise.
	(destroy_pieces): Remove extern "C" function declaration.
	(Pieces::~Pieces): Remove function declaration.
	(Pieces::operator=): Likewise.
	(Pieces::get_pieces): Handle changes to class fields.
	(Pieces::Pieces): Remove copy and move constructor declarations,
	adjust signature of remaining constructor declaration.
	(Pieces::pieces_vector): Remove member variable.
	(Pieces::handle): Likewise.
	(Pieces::data): Add member variable.
	* expand/rust-macro-builtins-asm.cc (expand_inline_asm_strings):
	Use references to avoid copying.

libgrust/ChangeLog:

	* libformat_parser/src/lib.rs (struct FFIVec): New.
	(trait StringLeakExt): Remove.
	(struct FFIOpt): New.
	(trait IntoFFI): Adjust implementation for Option.
	(struct RustHamster): Add lifetime and adjust conversion to and
	from &str.
	(enum Piece): Adjust definition to handle changes to
	RustHamster.
	(struct Argument): Likewise.
	(struct FormatSpec): Use FFIOpt and RustHamster.
	(enum Position): Use RustHamster.
	(enum Count): Likewise.
	(struct PieceSlice): Replace with...
	(typedef PieceVec): ...this.
	(struct RustString): Remove.
	(struct FormatArgsHandle): Likewise.
	(fn collect_pieces): Adjust signature, greatly simplifying
	implementation.
	(fn clone_pieces): Likewise.
	(fn destroy_pieces): Remove.
	(trait LayoutExt): New.
	(fn rust_ffi_alloc): New.
	(fn rust_ffi_dealloc): New.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>