Add validator grid mapper for neighbor discovery

[bamzedev]

Closes #230

Primarily for the purpose of block announcements, the previous, current, 
and next validator sets are conceptually arranged in a grid structure. 
Two validators are considered neighbours in the grid if:

They are validators in the same epoch and either have the same row (index / W) 
or the same column (index % W). W here is floor(sqrt(V)), where V is the number of validators.
They are validators in different epochs but have the same index.

[greywolve]

LGTM, only comment would be a randomized test would be nice too, but we can always add that later.