faster implementation of integer square root
* refactored lookahead code to compute everything in dda steps
* new 12bits fixed point implementation of 1/sqrt(x)
* replaced RAMPING iterative scheme with an approximation of the explicit formula (sqrt(n+1)-sqrt(n) ~ 1/(2*sqrt(n)))
NB: the 1/sqrt(x) implementation is a bit faster than a 32bits divide (it takes about 11% less time to complete) and could be even faster if one requires only 8bits
(precision starts getting poor for big values of n which are likely to be required by small acceleration values)