Stop assuming interval pointers and lisp objects can be distinguished by

Stop assuming interval pointers and lisp objects can be distinguished by
inspection.  Beginnings of support for expensive internal consistency checks.

* config.in (ENABLE_CHECKING): Undef.

* lisp.h (struct interval): Replace "parent" field with a union of interval
pointer and Lisp_Object; add new bitfield to use as discriminant.  Change other
flag fields to bitfields.
(CHECK): New macro for consistency checking.  If ENABLE_CHECKING is defined and
the supplied test fails, print a message and abort.
(eassert): New macro.  Use CHECK to provide an assert-like facility.

* intervals.h (NULL_INTERVAL_P): Now applies only to real interval pointers;
abort if the value looks like a lisp object.
(NULL_INTERVAL_P, NULL_PARENT, HAS_PARENT, HAS_OBJECT, SET_PARENT, SET_OBJECT,
INTERVAL_PARENT, GET_INTERVAL_OBJECT, COPY_PARENT): Modify for new interval
parent definition.

* alloc.c (mark_interval_tree, MARK_INTERVAL_TREE, UNMARK_BALANCE_INTERVALS):
Update references that need an addressable lisp object in the interval
structure.
(die): New function.
(suppress_checking): New variable.

* intervals.c (interval_start_pos): Just return 0 if there's no parent object.