Part 10 of proto array fork choice - Add Store

[terencechain]

This PR implements the store functions that satisfies fork choice interface:

type ForkChoice interface {
	HeadRetriever        // to compute head.
	BlockProcessor       // to track new block for fork choice.
	AttestationProcessor // to track new attestation for fork choice.
	Pruner               // to clean old data for fork choice.
}

Please don't be discourage with the diff, the implementation is just 80 lines that's in in store.go. Rest 800 lines are tests with clear commentary and text graphs 👍

[terencechain]

this looks good on my IDE, but looks messed up here