Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_shrink.c

@@ -0,0 +1,22 @@
+#include "tommath_private.h"
+#ifdef MP_SHRINK_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* shrink a bignum */
+mp_err mp_shrink(mp_int *a)
+{
+   int alloc = MP_MAX(MP_MIN_DIGIT_COUNT, a->used);
+   if (a->alloc != alloc) {
+      mp_digit *dp = (mp_digit *) MP_REALLOC(a->dp,
+                                             (size_t)a->alloc * sizeof(mp_digit),
+                                             (size_t)alloc * sizeof(mp_digit));
+      if (dp == NULL) {
+         return MP_MEM;
+      }
+      a->dp    = dp;
+      a->alloc = alloc;
+   }
+   return MP_OKAY;
+}
+#endif