Prevent keys with length >= 256

[mappum]

Merk does not support keys of 256 bytes or more, but this fails at the write step which means if an application tried to produce an oversized key then e.g. a whole block would fail. This should fail sooner by e.g. checking at the Store::put step, but ideally could be checked statically by keeping track of prefix lengths with e.g. a Key type (#8) (likely based on a fixed-size array rather than a Vec<u8>) with const generics, and maybe const generics for known min/max lengths of ed encodings (this change would maybe happen directly in the Encode and Decode traits).