Fuse unmount

[nvictus]

Allows FuseProcess to check if a desired mountpoint is already mounted, and allows the user to forcibly unmount and restart the process locally.

Produces a warning if fuse.start(tmp_dir) is called but {tmp_dir}/schemas is already mounted.

Also adds at-exit handler to make sure a FUSE subprocess spawned by this process is cleaned up.

[manzt]

Kind of curious if we should move the fuse stuff under something like experimental. It would be really great to have this sorted out before a v1, but I anticipate this API is the most likely to need changes in the future. Exposing it under an experimental namespace might give us more flexibility to do so (and not prevent us from dragging our toes on an official release).