Replaced fast mappings by even better ones

Replaced fast mappings by even better ones

Turned out there were much simpler ways to compute the mapping, thanks
to the fact that when the prime p is congruent to 5 modulo 8, we have
this nice equality:

    x^((p-5)/8) = sqrt(1/x)          if x is square,
    x^((p-5)/8) = sqrt(sqrt(-1)/x)   otherwise

The code was kindly given by Andrew Moon, who got the original trick
from Mike Hamburg.