Utility function mp_resize.

This reallocs an existing mp_int to have a different physical size,
e.g. to make sure there's enough space at the top of it.

Trivial, but I'm a little surprised I haven't needed it until now!

mpint.h

@@ -42,6 +42,13 @@ mp_int *mp_new(size_t maxbits);
 void mp_free(mp_int *);
 void mp_clear(mp_int *x);
 
+/*
+ * Resize the physical size of existing mp_int, e.g. so that you have
+ * room to transform it in place to a larger value. Destroys the old
+ * mp_int in the process.
+ */
+mp_int *mp_resize(mp_int *, size_t newmaxbits);
+
 /*
  * Create mp_ints from various sources: little- and big-endian binary
  * data, an ordinary C unsigned integer type, a decimal or hex string