Handle POST to /data/fs/[path]

Each line of the body is submitted to MongoDB as a "save" (which is effectively update/insert).

Given the MongoDB model, this actually supports creating a new collection, saving new documents to an existing collection, and updating existing documents (see the README).

All lines are parsed before any data is saved, so nothing is changed if the input is bad. However, partial writes can occur if the MongoDB driver gives an error.

Also added a command `save [path] [object]` to the REPL, and made the `ls` and `cd` commands actually work, for the most part.
See above for the rationale. `Path` is growing lots of operations, and continues to be a pain. For starters, it would be great if relative and absolute paths were distinct types.

Also I can't help but think that file system paths/URIs are a data type type that others must have wrestled with. Obviously we're not using them in exactly the usual way, but still. Is that worth digging around for?