spa: pod parser: Make `get_pod()` method safe, return safe &crate::pod::Pod instead of raw ptr, add Pod(_) variant to parser macro.

The sys method guarentees that it returns either an error or a valid pod that fits in the parsers memory slice, so we can safely convert the raw pod into the safe reference wrapper type and also make the function safe.

Also adds it to the parser_get!() macro.

(And a couple cosmetic fixes I ran into while working on this)