Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_2expt.c

@@ -0,0 +1,31 @@
+#include "tommath_private.h"
+#ifdef MP_2EXPT_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* computes a = 2**b
+ *
+ * Simple algorithm which zeroes the int, grows it then just sets one bit
+ * as required.
+ */
+mp_err mp_2expt(mp_int *a, int b)
+{
+   mp_err    err;
+
+   /* zero a as per default */
+   mp_zero(a);
+
+   /* grow a to accomodate the single bit */
+   if ((err = mp_grow(a, (b / MP_DIGIT_BIT) + 1)) != MP_OKAY) {
+      return err;
+   }
+
+   /* set the used count of where the bit will go */
+   a->used = (b / MP_DIGIT_BIT) + 1;
+
+   /* put the single bit in its place */
+   a->dp[b / MP_DIGIT_BIT] = (mp_digit)1 << (mp_digit)(b % MP_DIGIT_BIT);
+
+   return MP_OKAY;
+}
+#endif