Count bytes, not records, in specpdl_ref

Count bytes, not records, in specpdl_ref

SPECPDL_INDEX becomes much faster because the pointer subtraction,
expensive because of the inconveniently sized specpdl records, is now
gone.  This translates to a useful speedup on a wide variety of code.

* src/lisp.h (specpdl_ref, wrap_specpdl_ref, unwrap_specpdl_ref):
Rename member.
(specpdl_count_to_ref, specpdl_ref_to_count)
(specpdl_ref_add, specpdl_ref_to_ptr, SPECPDL_INDEX):
Count offset in bytes rather than in records.