`Default` was pointless, so it was replaced with internal
`from_raw_parts` method which also checks the length.
This commit also documents changes to `SerializedSignature`.
Closes#454
This de-clutters the code and prepares for the next step of adding
`IntoIterator`. The type is still re-exported so the change is neither
breaking nor inconvenient.
This also adds more datialed explanation of `SerializedSignature` and
why it's needed.
Martin Habovstiak