Speed up for D.d by ~10%

Modifications:
* removed some void initializers that had no impact on performance.
* random number generator now skips xor with 1 on overflow.
* D specific rearrangement of checkColl(), for better LLVM omptimization (-10% off total runtime for me).
* lines of code excluding empty and comment: 100

I kept close to the original in terms of the steps that are performed, but gave it some OOP style.