Commitment: combine `witnessComputeCellHashWithStorage` and its ancestor `computeCellHash`

So `computeCellHash` computes hash depending on cell contents. If the cell is actually a branch node (hashLen > 0) and represent a top branch of storage subtrie, it computes storage root. We need to distinguish this specific case and be able to return non nil storage root along with account cell hash.

- used in witnesses
- when we read from the trie, without that we do not know actual storage root of account cell, we can only catch it during folding.

To reduce this ambiguity optional field would be handy instead of 2 veryyy similar implementations.