Fallback code for two-word multiplication

[Torbjörn Granlund]

Dear Adrien,

It is hard for the GMP developers to assess your work, I'm afraid.
Timing tables of your work and GMP might lead to obvious conclusions for
you, but not to us.

The best way of helping to improve GMP is to make a proposed change to
the GMP code, then time the result using either our tune/speed program
or if necessary by writing your own timing code.

-- 
Torbjörn
Please encrypt, key id 0xC8601622