Fix ffi and enum conventions

Fix ffi and enum conventions

gcc/rust/ChangeLog:

	* ast/rust-fmt.h (enum ParseMode):
	Drop typedef in Cpp

libgrust/ChangeLog:

	* libformat_parser/generic_format_parser/src/lib.rs:
	Remove repr(C)
	* libformat_parser/src/bin.rs: Use ffi
	* libformat_parser/src/lib.rs: pub ffi, create ParseMode and match
	rustc's parse mode