Use libspa::pod::Pod where applicable

The simple stream API is a way to create a stream from just a mainloop. Context and Core are handled automatically internally.

&libspa::pod::Pod is a more specific type than &[u8] and ensures it represents a valid pod. At the same time, it offers more safety and is easier to use than *const spa_sys::spa_pod.

Also adds from_bytes(), as_bytes(), type_(), size() methods to the pod type.

I also added redefinitions for spa_debug_pod() and related functions as I was going to add a Debug impl based on it, but this turns out to be more complicated than I initially thought, as it is hard to get the functions to write to the formatter instead of stdout, and it is also not possible to automatically select the correct spa_types table.