Make live_*_p more accurate

Make live_*_p more accurate

* src/alloc.c (live_string_holding, live_cons_holding)
(live_symbol_holding, live_vector_holding):
Return a C pointer, not a Lisp_Object.  All callers changed.
This helps the compiler a bit.
(live_string_p, live_cons_p, live_symbol_p, live_vector_p):
Require that P point directly at the object, rather than
somewhere within the object.  This fixes some false positives
with valid_lisp_object_p (used only in debugging).
(mark_maybe_object): Rely on the new accuracy.