gccrs: proc_macro: Increase FFIString usage

Two remaining structures in the rust interface were still using raw
string pointer and length couples to communicate with the C++ library
throught extern C functions. Using FFIString instead allow us to reduce
the scope of potential errors using those raw pointers. As FFIString
encapsulate raw pointer operations there will be only one locaiton to
look after.

libgrust/ChangeLog:

	* libproc_macro/rust/bridge/literal.rs: Change extern C
	function argument from raw string pointer and length to
	FFIString.
	* libproc_macro/rust/bridge/token_stream.rs: Likewise.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>