Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_grow.c

@@ -0,0 +1,40 @@
+#include "tommath_private.h"
+#ifdef MP_GROW_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* grow as required */
+mp_err mp_grow(mp_int *a, int size)
+{
+   /* if the alloc size is smaller alloc more ram */
+   if (a->alloc < size) {
+      mp_digit *dp;
+
+      if (size > MP_MAX_DIGIT_COUNT) {
+         return MP_OVF;
+      }
+
+      /* reallocate the array a->dp
+       *
+       * We store the return in a temporary variable
+       * in case the operation failed we don't want
+       * to overwrite the dp member of a.
+       */
+      dp = (mp_digit *) MP_REALLOC(a->dp,
+                                   (size_t)a->alloc * sizeof(mp_digit),
+                                   (size_t)size * sizeof(mp_digit));
+      if (dp == NULL) {
+         /* reallocation failed but "a" is still valid [can be freed] */
+         return MP_MEM;
+      }
+
+      /* reallocation succeeded so set a->dp */
+      a->dp = dp;
+
+      /* zero excess digits */
+      s_mp_zero_digs(a->dp + a->alloc, size - a->alloc);
+      a->alloc = size;
+   }
+   return MP_OKAY;
+}
+#endif