vfs: flatten resolution cache key

This will require more storage if there are a lot of lookups with the
same parent and differing paths, but otherwise, without more
intentional structure sharing among paths, this should be better, and
we can always revisit the arrangement later.

Serializing the parent path segments should also make sure the same
parent (semantically-speaking) contributes the same hash to the key.
Previously, Metadata objects could prevent that, given their trivial,
pointer-based hashes.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>