gccrs: libproc_macro: Add literal ffi implementations

gccrs: libproc_macro: Add literal ffi implementations

Add literal type related ffi functions implementation.

libgrust/ChangeLog:

	* libproc_macro/literal.h (Literal__drop): Implement
	drop.
	(Literal__string): Constructor from a string function prototype.
	(Literal__byte_string): Constructor from a byte string function
	prototype.
	(Literal__from_string): Add function body, no implementation
	yet.
	* libproc_macro/literal.cc: New file.

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