Cleanup BasicBlock constructors

[Stebalien]

1. NewBlock should take a cid.Prefix.
2. NewBlockWithCid should not return an error, it should panic if the CID doesn't match and we're in debug mode.

This may cause a bit of code churn...

[gammazero]

NewBlock should take a cid.Prefix.

To avoid widespread breakage across many repos, we should use a new function or an option argument.

NewBlockWithCid should not return an error,

Changing this will cause breakage in many repos. We could justify returning the error as it allows for some future error when some bad CID is used. We could also return and error if the CID is undefined.