Turn specpdl_ref into a type of its own

Turn specpdl_ref into a type of its own

Wrap the specpdl_ref contents in a struct, which gives us some very
useful type checking to make sure that all code that assumed integral
specpdl refs has been converted.

We only do this on 64-bit platforms, since those tend to have modern
ABIs where small structs are optimised as scalars.  In other words,
this change should not affect the compiled code.

* src/lisp.h (specpdl_ref): Now a struct on 64-bit platforms.
(wrap_specpdl_ref, unwrap_specpdl_ref): New.
(specpdl_count_to_ref, specpdl_ref_to_count)
(specpdl_ref_eq, specpdl_ref_lt)
(specpdl_ref_valid_p, make_invalid_specpdl_ref): Use new wrappers.