[prototype] Map key redesign

[uint]

Closes #48

This whole monstrosity:

• enables bounded iteration for maps, but only if there's no length-prefixing of the key going on
• only length-prefixes keys when the key is dynamically sized AND it's not the final "component" of the complete key (as stored in the backend)
• implements numerical keys for maps

TODO:

• simplify, rearrange, rename things, make everything easier to understand for future maintainers
• internal documentation
• review what should be public
• API reference
• complete tests
• check if Key/OwnedKey implementors can receive something like &[u8; 4] when they specify their key as having 4 bytes, and &[u8] when they specify the key as dynamically sized - less custom error handling needed that way
  not easy: issue created - OwnedKey::from_bytes should operate on things like &[u8; 4] when key size is fixed #58
• look for other std types to implement OwnedKey/Key for
• can this kind of stuff help make key types we get from iteration a little less crazy?
  separate issue: Experiment: can we improve key types we get from map iteration? #59