Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_mul_2.c

@@ -0,0 +1,53 @@
+#include "tommath_private.h"
+#ifdef MP_MUL_2_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* b = a*2 */
+mp_err mp_mul_2(const mp_int *a, mp_int *b)
+{
+   mp_err err;
+   int x, oldused;
+   mp_digit r;
+
+   /* grow to accomodate result */
+   if ((err = mp_grow(b, a->used + 1)) != MP_OKAY) {
+      return err;
+   }
+
+   oldused = b->used;
+   b->used = a->used;
+
+   /* carry */
+   r = 0;
+   for (x = 0; x < a->used; x++) {
+
+      /* get what will be the *next* carry bit from the
+       * MSB of the current digit
+       */
+      mp_digit rr = a->dp[x] >> (mp_digit)(MP_DIGIT_BIT - 1);
+
+      /* now shift up this digit, add in the carry [from the previous] */
+      b->dp[x] = ((a->dp[x] << 1uL) | r) & MP_MASK;
+
+      /* copy the carry that would be from the source
+       * digit into the next iteration
+       */
+      r = rr;
+   }
+
+   /* new leading digit? */
+   if (r != 0u) {
+      /* add a MSB which is always 1 at this point */
+      b->dp[b->used++] = 1;
+   }
+
+   /* now zero any excess digits on the destination
+    * that we didn't write to
+    */
+   s_mp_zero_digs(b->dp + b->used, oldused - b->used);
+
+   b->sign = a->sign;
+   return MP_OKAY;
+}
+#endif