Replace fixed and variable length interfaces with usage of a trait #86
Description
We currently have a good deal of duplication on the library boundary to allow for users to use fixed length arrays and boundedvecs for their keys. In #84 I replaced replaced the fixed-length implementations to just delegate to the variable length methods, however this still leaves us with a larger than necessary interface.
As we only use these keys to calculate the keyhash from them, we could make these methods generic over a KeyHashable (name just for example) trait which defines how to calculate the keyhash from a type. We could then implement this for a few types, e.g. array, boundedvec, string. to get more flexibility but without a huge explosion of methods on the interface.