Use the Genesis state as a Trusted State in the light client

[yihuang]

It seems verify_single need a TrustedState from previous block, TrustedState contains a signed header and validator set, but when verifying the first block, there is no previous signed header.

Maybe we should make TrustedState into (Option<SignedHeader>, ValidatorSet), then genesis trusted state would be (None, genesis validator set).

[tomtau]

#186

[ebuchman]

Great point. We'll need to ensure you can use the genesis as a TrustedState somehow.

The light client API is undergoing pretty heavy restructuring, but will make sure this gets addressed.

Essentially the problem is that the Genesis State does not contain a header, though it does have a timestamp and an initial validator set. We should still be able to initialize from it - we can consider it height 0.

[OStevan]

Container issue is somewhat related with this.

[brapse]

This blocked by specification work, perhaps @josef-widder can help here.