div_qr_1 interface

[Torbjorn Granlund]

I pushed initial C versions of these functions:

    mpn_div_qr_1n_pi2
    mpn_div_qr_1u_pi2

I have had these for a long time, judging from the file time stamps.

These accept n-limb dividends in a single consecutive operand and
generate n-limb quotients also in a consecutive operand.  I now realise
that you use a n+1 limb dividend passed as separate operands for the low
n limbs and the high limb.  I had misunderstood your intended
interface...

The new pi2 functions follow your suggestion to stuff the dividend
together with the various precomputations in a single operand.  Here it
is a structure, perhaps we could use a simple array instead (like cps).

-- 
Torbjörn