Expose complete prototype(s)

This PR takes us to a full prototype expose on `exports`, no more hiding data, anything "private" is prefixed with `_` and is caveat emptor. ReadStream and WriteStream are also now exposed with the `create()` method removed.

The reason for this is twofold:

 * Simplify the implementation and make it more accessible to newbs to understand and contribute to.
 * Allow for reuse, there's a lot in LevelUP that could be reused, perhaps even for something like sublevel.