Minor fixups for intmax_t→mpz_t conversion

Minor fixups for intmax_t→mpz_t conversion

* src/alloc.c (mpz_set_intmax_slow): Tighten assertion.
Work even in the unlikely case where libgmp uses nails.
* src/data.c (FIXNUMS_FIT_IN_LONG): New constant.
(arith_driver): Use it to tighten compile-time checks.
* src/lisp.h (mpz_set_intmax): Do not assume that converting
an out-of-range value to ‘long’ is harmless, as it might raise
a signal.  Use simpler expression; compiler can optimize.