gccrs: libproc_macro: Add Span definition

Add Span type definition in the rust interface.

libgrust/ChangeLog:

	* libproc_macro/rust/bridge/group.rs: Add span member
	to the Group structure.
	* libproc_macro/rust/bridge/ident.rs: Likewise with the
	Ident structure.
	* libproc_macro/rust/bridge/literal.rs: Likewise with
	the Literal structure.
	* libproc_macro/rust/bridge/punct.rs: Likewise with the
	Punct structure.
	* libproc_macro/rust/bridge/span.rs: Add span
	internals.

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