Extended precision library

[G.E.]

Hello,
Sure good idea, that's fairly simple.

List of changes to do :
- add an xversion function
- use Newton methode for division
- change xinit so that it takes a number of digits and not slices
- add cos, asin, acos
- normalize angles as input to sin/cos/tan
- (hard) fix precision issues in cordic functions

G.E.