load methods: support memoryview additionally to bytes #201

Similar to #200, but for the return value of the load methods.

We would need to unit-test and document this for the return types of:

• Store.load(name)
• backend.load(name) for all backends

This is slightly more problematic than the store methods, because the caller then needs to be able to deal with memoryview or bytes (we can't just handle this internally).

Ideas:

• it could just be documented that the return value might be memoryview or bytes - then it is just the caller's task to handle this. Could lead to rather ugly boilerplate in the caller code.
• or there could be a flag in the Store and in the backends whether they are allowed to return both types or just bytes.