Use Option<TCid> for previous checkpoint in CheckData

[adlrocha]

In #79 we had to introduce the use of a genesis cid for the first checkpoint committed to avoid creating a corrupted state in the VM.

A better fix would be to use an Option<TCid<TLink<Checkpoint>> for that field in Checkpoints. This would require a larger refactor, and as we are expecting the format of checkpoints to change when we introduce cross-net messages, it is better to defer the fix for then:

ipc-actors/gateway/src/checkpoint.rs

Line 122 in f5e54ad

pub prev_check: TCid<TLink<Checkpoint>>,