New mpint function mp_get_integer().

If you have an mp_int that you know will fit in an ordinary integer
type, this function gives it to you in that form.

mpint.c

@@ -257,6 +257,18 @@ unsigned mp_get_bit(mp_int *x, size_t bit)
                 (bit % BIGNUM_INT_BITS));
 }
 
+uintmax_t mp_get_integer(mp_int *x)
+{
+    uintmax_t toret = 0;
+    for (size_t i = x->nw; i-- > 0 ;) {
+        /* Shift in two stages to avoid undefined behaviour if the
+         * shift count equals the integer width */
+        toret = (toret << (BIGNUM_INT_BITS/2)) << (BIGNUM_INT_BITS/2);
+        toret |= x->w[i];
+    }
+    return toret;
+}
+
 void mp_set_bit(mp_int *x, size_t bit, unsigned val)
 {
     size_t word = bit / BIGNUM_INT_BITS;

mpint.h

@@ -100,6 +100,12 @@ mp_int *mp_power_2(size_t power);
 uint8_t mp_get_byte(mp_int *x, size_t byte);
 unsigned mp_get_bit(mp_int *x, size_t bit);
 
+/*
+ * Retrieve the value of an mp_int as a uintmax_t, assuming it's small
+ * enough to fit.
+ */
+uintmax_t mp_get_integer(mp_int *x);
+
 /*
  * Set an mp_int bit. Again, the bit index is not considered secret.
  * Do not pass an out-of-range index, on pain of assertion failure.