fix memory leak with bignums

Letting the gmp library use uncollectable allocations leads to
    leaks, if the bignums are not freed with mpz_clear. Since this
    can't be done without a finalizer, we have to use the standard
    allocation mechanism. Fixes #433.

src/c/big.d

@@ -273,13 +273,13 @@ _ecl_fix_divided_by_big(cl_fixnum x, cl_object y)
 static void *
 mp_alloc(size_t size)
 {
-  return ecl_alloc_uncollectable(size);
+  return ecl_alloc_atomic(size);
 }
 
 static void
 mp_free(void *ptr, size_t size)
 {
-  ecl_free_uncollectable(ptr);
+  ecl_dealloc(ptr);
 }
 
 static void *