* improved integer sqrt routine (I benchmarked an 8x improvement on my Atmega644 over the previous implementation)
* I rewrote LOOKAHEAD ramps calculations to store states as dda steps instead of feedrates, it avoids computing a few int_sqrt
* the RAMPING algorithm suffers from instabilities with LOOKAHEAD enabled, it could result in extremely slow moves.
  This problem occurs quickly with LOOKAHEAD (in my tests anyway) but it may also happen in the normal case (but I didn't notice it before).
  I solved the issue with a forced reinitialization of the move_state.c/n variables at the beginning of each move (yes it involves costly evaluation of c/n in LOOKAHEAD code but it works)