PSSH decoding probably got broken in some refactoring. system id is 16 bytes,
also scalar mapper should use cloned reader to not affect reader which in the case
broke reading system id bytes.
Add test with playread and widevine pssh boxes
Related to #282
Remove bitio.Buffer layer. bitio.Buffer was a kitchen sink layer with helpers
now it's just a buffer and most functions have been moved to decode instead.
bitio package now only have primitive types and functions simialar to standard
library io and bytes packages.
Make nearly eveything internally use bitio.Bit* interfaces so that slicing work
correctly this will also make it possible to start experimenting with more
complicated silcing helpers, ex things like:
breplace(.header.bitrate; 123) to get a new buffer with bitrate changed.
