Avoid libgmp aborts by imposing limits

Avoid libgmp aborts by imposing limits

libgmp calls ‘abort’ when given numbers too big for its
internal data structures.  The numeric limit is large and
platform-dependent; with 64-bit GMP 6.1.2 it is around
2**2**37.  Work around the problem by refusing to call libgmp
functions with arguments that would cause an abort.  With luck
libgmp will have a better way to do this in the future.
Also, introduce a variable integer-width that lets the user
control how large bignums can be.  This currently defaults
to 2**16, i.e., it allows bignums up to 2**2**16.  This
should be enough for ordinary computation, and should
help Emacs to avoid thrashing or hanging.
Problem noted by Pip Cet (Bug#32463#71).
* doc/lispref/numbers.texi, etc/NEWS:
Document recent bignum changes, including this one.
Improve documentation for bitwise operations, in the light
of bignums.
* src/alloc.c (make_number): Enforce integer-width.
(integer_overflow): New function.
(xrealloc_for_gmp, xfree_for_gmp):
Move here from emacs.c, as it's memory allocation.
(init_alloc): Initialize GMP here, rather than in emacs.c.
(integer_width): New var.
* src/data.c (GMP_NLIMBS_MAX, NLIMBS_LIMIT): New constants.
(emacs_mpz_size, emacs_mpz_mul)
(emacs_mpz_mul_2exp, emacs_mpz_pow_ui): New functions.
(arith_driver, Fash, expt_integer): Use them.
(expt_integer): New function, containing integer code
that was out of place in floatfns.c.
(check_bignum_size, xmalloc_for_gmp): Remove.
* src/emacs.c (main): Do not initialize GMP here.
* src/floatfns.c (Fexpt): Use expt_integer, which
now contains integer code moved from here.
* src/lisp.h (GMP_NUMB_BITS): Define if gmp.h doesn’t.