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.
Was already handled in fq in various places as gojq uses them
Update msgpack to support negative integers that can't represented as int64
Rename read try* number functions to make them more explicit
Move scalar into own package.
Split scalar code into decode related scalar code (that reads etc) and
scalar code that just transform the scalar value.
Use a scalar.Mapper interface instead of just a function.
Make mappers, assert and validat impement the interface.
