Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/s_mp_mul.c

@@ -0,0 +1,61 @@
+#include "tommath_private.h"
+#ifdef S_MP_MUL_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* multiplies |a| * |b| and only computes upto digs digits of result
+ * HAC pp. 595, Algorithm 14.12  Modified so you can control how
+ * many digits of output are created.
+ */
+mp_err s_mp_mul(const mp_int *a, const mp_int *b, mp_int *c, int digs)
+{
+   mp_int  t;
+   mp_err  err;
+   int     pa, ix;
+
+   /* can we use the fast multiplier? */
+   if ((digs < MP_WARRAY) &&
+       (MP_MIN(a->used, b->used) < MP_MAX_COMBA)) {
+      return s_mp_mul_comba(a, b, c, digs);
+   }
+
+   if ((err = mp_init_size(&t, digs)) != MP_OKAY) {
+      return err;
+   }
+   t.used = digs;
+
+   /* compute the digits of the product directly */
+   pa = a->used;
+   for (ix = 0; ix < pa; ix++) {
+      int iy, pb;
+      mp_digit u = 0;
+
+      /* limit ourselves to making digs digits of output */
+      pb = MP_MIN(b->used, digs - ix);
+
+      /* compute the columns of the output and propagate the carry */
+      for (iy = 0; iy < pb; iy++) {
+         /* compute the column as a mp_word */
+         mp_word r = (mp_word)t.dp[ix + iy] +
+                     ((mp_word)a->dp[ix] * (mp_word)b->dp[iy]) +
+                     (mp_word)u;
+
+         /* the new column is the lower part of the result */
+         t.dp[ix + iy] = (mp_digit)(r & (mp_word)MP_MASK);
+
+         /* get the carry word from the result */
+         u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
+      }
+      /* set carry if it is placed below digs */
+      if ((ix + iy) < digs) {
+         t.dp[ix + pb] = u;
+      }
+   }
+
+   mp_clamp(&t);
+   mp_exch(&t, c);
+
+   mp_clear(&t);
+   return MP_OKAY;
+}
+#endif