correctly rounded sqrt

[Szabolcs Nagy <nsz@port70.net>]

i tried to solve the sqrt issue with a wrapper
as discussed on irc, but i couldn't find a nice
solution

if we check |x - y*y| and try to figure out
the rounding from that, then there are two
approaches

1) use the veltkamp-dekker exact multiplication formulas
(sq1 algorithm in my code)

or

2) use simpler method for y*y and use argumentum reduction
to handle large/small numbers (x*=2^2n, y*=2^-n)


i tried both: 1) only works correctly for subnormals
with double precision evaluation so it needs arg reduction
2) does not work in all cases (|x-y*y| and |x-y1*y1|
might be the same and resolving the tie is nontrivial)

so 1) with arg reduction is my current solution
and i don't know if it works in all cases
..and the code might be slow