ses all memory, so
     there's an allocate and free every time a top-level function using
     <code>TMP</code> is called.  Would need
     <code>mp_set_memory_functions</code> to tell tal-notreent.c to release
     any cached memory when changing allocation functions though.
<li> <code>__gmp_tmp_alloc</code> from tal-notreent.c could be partially
     inlined.  If the current chunk has enough room then a couple of pointers
     can be updated.  Only if more space is required then a call to some sort
     