Fix bignum initialization

Fix bignum initialization

Problem reported by Andy Moreton in:
https://lists.gnu.org/r/emacs-devel/2018-09/msg00072.html
and crystal-ball diagnosis by Eli Zaretskii in:
https://lists.gnu.org/r/emacs-devel/2018-09/msg00075.html
* src/alloc.c (xrealloc_for_gmp, xfree_for_gmp): Move to bignum.c.
(init_alloc): Move bignum initialization to init_bignum.
* src/bignum.c (init_bignum): Rename from init_bignum_once.
All users changed.
* src/emacs.c (main): Call init_bignum after init_alloc,
instead of calling init_bignum_once after init_bignum.