The new coreiface.Path maps a path to the cid.Cid
resulting from a full path resolution.

The path is internally represented as a go-ipfs/path.Path,
but that doesn't matter to the outside.

Apart from the path-to-CID mapping, it also aims to hold all
resolved segment CIDs of the path. Right now it only exposes
Root(), and only for flat paths a la /ipfs/Qmfoo. In other cases,
the root is nil.

In the future, resolution will internally use
go-ipfs/path.Resolver.ResolvePathComponents and thus always return
the proper resolved segments, via Root(), or a future Segments() func.

- Add coreiface.Path with Cid() and Root().
- Add CoreAPI.ResolvePath() for getting a coreiface.Path.
- All functions now expect and return coreiface.Path.
- Add ParsePath() and ParseCid() for constructing a coreiface.Path.
- Add coreiface.Node and Link which are simply go-ipld-node.Node and Link.
- Add CoreAPI.ResolveNode() for getting a Node from a Path.

License: MIT
Signed-off-by: Lars Gierth <larsg@systemli.org>