Tidy up overrides and add watch API

[LukeSheard]

Following reading #89 I thought I'd PR two things:

1. Tidy up some of the overrides to make it neater
2. Add a watch API which watches all the filesystems. I do this by returning a proxy object around the FS.FSWatcher which calls all the watchers we get back from the underlying filesystems.

[streamich]

Thanks! This looks great, just added two small requests.

Also, are there any breaking changes in this PR? (I guess the behaviour of watch() changes.)

[LukeSheard]

I've done the changes you asked for. There's no breaking changes AFAIK here, just a what I would call a "fix" for #89 to support watch.

EDIT: I actually was reading https://nodejs.org/docs/latest/api/fs.html#fs_fs_unwatchfile_filename_listener and now added a method which throws an error when that is called - since we don't track watchers to be able to call watcher.stop() and the original implementation doesn't work anyway. So I would call this a breaking change.

[LukeSheard]

Realized my mistake in my latest push that broke the build in switching to Set.

This should be good to merge now.

[streamich]

Thanks! LGTM

[wood1986]

@LukeSheard Did you test if it throws an exception when an invalid is provided
See streamich/memfs#859