Basic support, can also do CER and DER but without any extra validation.
No schema support.
Redo format doc.md usage a bit, now format/<dir>/<format>.md instead.
Related to #20
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.