This adds support for the x format code in struct.pack and struct.unpack.
The primary use case for this is ignoring bytes while unpacking. When
interfacing with existing systems, it may often happen that you either have
fields in a struct that aren't properly specified or you simply don't care
about them. Being able to easily skip them is useful.
Signed-off-by: Daniël van de Giessen <daniel@dvdgiessen.nl>
