Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_mul_d.c

@@ -0,0 +1,68 @@
+#include "tommath_private.h"
+#ifdef MP_MUL_D_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* multiply by a digit */
+mp_err mp_mul_d(const mp_int *a, mp_digit b, mp_int *c)
+{
+   mp_digit u;
+   mp_err   err;
+   int   ix, oldused;
+
+   if (b == 1u) {
+      return mp_copy(a, c);
+   }
+
+   /* power of two ? */
+   if (MP_HAS(MP_MUL_2) && (b == 2u)) {
+      return mp_mul_2(a, c);
+   }
+   if (MP_HAS(MP_MUL_2D) && MP_IS_2EXPT(b)) {
+      ix = 1;
+      while ((ix < MP_DIGIT_BIT) && (b != (((mp_digit)1)<<ix))) {
+         ix++;
+      }
+      return mp_mul_2d(a, ix, c);
+   }
+
+   /* make sure c is big enough to hold a*b */
+   if ((err = mp_grow(c, a->used + 1)) != MP_OKAY) {
+      return err;
+   }
+
+   /* get the original destinations used count */
+   oldused = c->used;
+
+   /* set the sign */
+   c->sign = a->sign;
+
+   /* zero carry */
+   u = 0;
+
+   /* compute columns */
+   for (ix = 0; ix < a->used; ix++) {
+      /* compute product and carry sum for this term */
+      mp_word r       = (mp_word)u + ((mp_word)a->dp[ix] * (mp_word)b);
+
+      /* mask off higher bits to get a single digit */
+      c->dp[ix] = (mp_digit)(r & (mp_word)MP_MASK);
+
+      /* send carry into next iteration */
+      u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
+   }
+
+   /* store final carry [if any] and increment ix offset  */
+   c->dp[ix] = u;
+
+   /* set used count */
+   c->used = a->used + 1;
+
+   /* now zero digits above the top */
+   s_mp_zero_digs(c->dp + c->used, oldused - c->used);
+
+   mp_clamp(c);
+
+   return MP_OKAY;
+}
+#endif