We've a specific type for object paths but for convenience we've been
allowing string types to be passed as object paths that we internally
convert to ObjectPath or OwnedObjectPath. However, we've been
inconsisten on this practice and require ObjectPath to be passed by the
user in some places.

This commit attempts to make the API consisten w/o dropping the
convenience. We now expect users to always pass something that can be
converted into an ObjectPath, which includes both &str and String.

API BREAK!!
===========

While a lot of existing code (especially client-side) will continue to
build and work against this commit, this is very much an API break.