gccrs: libproc_macro: Change rust literal internals

gccrs: libproc_macro: Change rust literal internals

Rust interface literal internals were taking a type and storing that
type. This lead to multiple problems such as various conversion from
string to int/float/other type as well as dead end on undetermined types
(type checker runs at a later stage).

libgrust/ChangeLog:

	* libproc_macro/rust/bridge.rs: Add ffistring module.
	* libproc_macro/rust/bridge/literal.rs: Rework type
	internals.
	* libproc_macro/rust/bridge/ffistring.rs: New file.

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