Pi_Chud Demo, mpf_out_str, etc

[Torbjorn Granlund]

I wrote:

  Both mpf_get_str and mpf_out_str used to need runtime quadratic to the
  number of digits produced.  Now it is O(M(n)).

I suppose that's not really true, the mp*_get_str and consequently
the mp*_out_str functions rely on division, which is O(M(n)log(n)).

-- 
Torbjörn