Maybe replace Serve type switch with an interface that Read calls methods of based on message type

[tv42]

See #142 / bd00642 for inspiration. Basically a flat (non-Node/Handle-based) interface that gets methods called, that looks up the right Node/Handle and delegates.

Downsides: how does this interact with vmsplice (#35)? Might end up pushing the goroutine management from fuse/fs to fuse, which is not desirable at all; fuse was supposed to be just low-level wire protocol understanding. Maybe fuse calls those methods sequentially for every message in the buffer, and the methods themselves handle starting goroutines and refcounting?