The parser for a given object is already known from the object itself and
particularly must never change. Reassigning the value in `uri_unserialize()` is
just unsafe, especially since the existing `->uri` is freed with the destructor
of the reassigned parser.

Just rely on the `->parser` field being set to the correct value.

Similarly to the previous change to `uri_unserialize()`, the `->parser` must
always match the object for the freeing to be safe.

Given that we expect to successfully parse URIs, we can eagerly initialize the
resulting URI object when using the `::parse()` methods and destruct it again
when parsing fails and `null` is returned instead. Calling the destructor is
safe, since `uri` will be `NULL`, which will result in a noop.

The `base_url_object` must also match the object that is currently being
constructed. Verify this using assertions matching the `->ce` and the
`->parser`.