Add new `(C-not implementable)` tag on `EventsProvider`

This makes it so that users cannot usefully implement their own
`EventsProvider`, which would require substantial new logic in the
bindings generator (for generic methods). In the case of
`EventsProvider`, because there are no Rust methods which accept an
`EventsProvider` as an argument, this is perfectly OK as the
generated code would be entirely unused anyway.