Preparation to make decoder use less memory and API more type safe.
Now each scalar type has it's own struct type so it can store different
things and enables to have a scalar interface.
Also own types will enable experimenting with decode DLS designs like
using chained methods that are type aware.
Refactor to use groups between network layers to make them less coupled and reusable:
link_frame (ethernet etc)
inet_packet (ipv4,ipv6 etc)
ip_packet (tcp packet (not stream), udp segment etc)
Rename data to payload as i think it makes more sense for network data
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.
