Turn it into the `estimateBlockSize` method of `SerialiseNodeToNodeConstraints`.
Add `estimateHfcBlockSize` to `SerialiseHFC` so that we can provide a default
instance for the HFC which can still be overridden.

Split off the remaining methods from `RunNode` into `NodeInitStorage` in the new
`Ouroboros.Consensus.Node.InitStorage` module.

Introduce (yet) another config: the `StorageConfig`, used by the methods of
`NodeInitStorage`. Reason: we can't project a HFC `TopLevelConfig` to the
`TopLevelConfig` from one era because of the partial consensus and ledger
configs, but we *can* do that for a `StorageConfig`. This makes it possible to
implement `NodeInitStorage` for the HFC.

Since we have an `NodeInitStorage` instance for the HFC, we can give one for
`RunNode` too instead of having to provide it for each HFC instantiation! Our
sin bin is finally empty. End goal achieved 😎

Merge `checkEmpty` and `addBlock` in `addBlockIfEmpty`. This is currently the
only use of this API and merging them expresses the intent better. Moreover,
this avoids the racy nature of the two (although there will be no race condition
in practice).