Consistent `*_raw` methods

Every wrapper (owned or unowned) should have a as_raw(&self) -> *mut <c type> method, and owned wrappers also from_raw(*mut <c type>) -> Self and into_raw(self) -> *mut <c type> methods for use with -sys functions and other libraries.

Right now, this is not done consistently, for example pipewire::Core has none of these.