- Separate FileMount into separate FileMount and WritableFileMount
classes. This separates the (relatively simple) read-only code from
the (soon to be even more complex) read/write code.
It also allows you to create read-only mounts which don't bother with
filesystem accounting, which is nice.
- Make openForWrite/openForAppend always return a SeekableFileHandle.
Appendable files still cannot be seeked within, but that check is now
done on the FS side.
- Refactor the various mount tests to live in test contract interfaces,
allowing us to reuse them between mounts.
- Clean up our error handling a little better. (Most) file-specific code
has been moved to FileMount, and ArchiveMount-derived classes now
throw correct path-localised exceptions.
