Large operand truncating division code

[Torbjorn Granlund]

The section on truncating division on the GMP Tidbits page now has
code for truncating inversion and truncating division.

The code uses the same variant of Barrett's algorithm as the divexact
code at the same page, except that the truncating code works left-to-
right, whereas the divexact code works right-to-left.

The algorithm, as mentioned before, uses an inverse of a smaller size
than in the standard Barrett algorithm.

-- 
Torbjörn