Prepare to allow more complex expressions.

All DetachedSong instances should be converted to LightSong first,
which is a very cheap operation.

Will allow more complex fitler expression, such as negation (#89).

Not yet wired to the protocol.

Having this as one-byte integer actually adds overhead and saves no
memory due to padding.

Fixes search within mount points, resulting in error "No such
directory".

Closes #190

Closes #172

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Error message was 'too few arguments for "find"' because the "base"
constraint was not supported, and no other constraints remained.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.