Extending minimal sharding with cross-links

[tim]

@vbuterin

suppose that the last main-chain block when a collation for that shard was linked is T. Let R[i] be randomness from block i. For any block S > T, find the most recent block B such that B.number - T is an exact power of 2 (possibly with some minimum, eg. 128).

ahh, this is my kind of sentence. so concise