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.
