Take advantage of manifests and the squash down

[vweevers]

Dumping some thoughts. A couple of efforts are underway and relevant to subleveldown:

1. Add manifests (Add manifest to levelup and abstract-leveldown implementations community#83)
2. Add type (Add type property to levelup and abstract-leveldown implementations community#82)
3. The squash down (let's give it a name) (Merge levelup into abstract-leveldown community#58)

Let's assume that somewhere in the future, abstract-leveldown has reached feature parity with levelup. Then subleveldown won't need to unwrap levelup and deferred-leveldown, nor will it need to rewrap. Issues like #60 will be gone.

Whether it will need to unwrap/rewrap encoding-down is an open question. In any case we'll have more options to feature-detect: subleveldown might detect db.type === 'encoding-down' or db.supports.encodings.

In the latter case maybe subleveldown will be able to "punch through" the encodings of its input db by using { keyEncoding: 'binary' } internally, and still support encodings externally (assuming that'll be an abstract-leveldown feature). That would be quite nice.

[vweevers]

Encodings ended up builtin to abstract-level which greatly simplified subleveldown, to the point that subleveldown could also be merged into abstract-level: Level/abstract-level#8.