Change the compiler's big integer library to use libTomMath

This now replaces Bill's crappy big int implementation

src/libtommath/mp_fread.c

@@ -0,0 +1,66 @@
+#include "tommath_private.h"
+#ifdef MP_FREAD_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+#ifndef MP_NO_FILE
+/* read a bigint from a file stream in ASCII */
+mp_err mp_fread(mp_int *a, int radix, FILE *stream)
+{
+   mp_err err;
+   mp_sign sign = MP_ZPOS;
+   int ch;
+
+   /* make sure the radix is ok */
+   if ((radix < 2) || (radix > 64)) {
+      return MP_VAL;
+   }
+
+   /* if first digit is - then set negative */
+   ch = fgetc(stream);
+   if (ch == (int)'-') {
+      sign = MP_NEG;
+      ch = fgetc(stream);
+   }
+
+   /* no digits, return error */
+   if (ch == EOF) {
+      return MP_ERR;
+   }
+
+   /* clear a */
+   mp_zero(a);
+
+   do {
+      uint8_t y;
+      unsigned pos;
+      ch = (radix <= 36) ? MP_TOUPPER(ch) : ch;
+      pos = (unsigned)(ch - (int)'+');
+      if (MP_RADIX_MAP_REVERSE_SIZE <= pos) {
+         break;
+      }
+
+      y = s_mp_radix_map_reverse[pos];
+
+      if (y >= radix) {
+         break;
+      }
+
+      /* shift up and add */
+      if ((err = mp_mul_d(a, (mp_digit)radix, a)) != MP_OKAY) {
+         return err;
+      }
+      if ((err = mp_add_d(a, y, a)) != MP_OKAY) {
+         return err;
+      }
+   } while ((ch = fgetc(stream)) != EOF);
+
+   if (!mp_iszero(a)) {
+      a->sign = sign;
+   }
+
+   return MP_OKAY;
+}
+#endif
+
+#endif