More acceleration maths.

* Refactored lookahead code to compute everything in dda steps.

* New 12 bits 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 32 bits divide
    (it takes about 11% less time to complete) and could be even faster if
    one requires only 8 bits. Also, precision starts getting poor for big
    values of n which are likely to be required by small acceleration values.