In a few specific portions of the spec format it's convenient to have
access to some of the functions defined in the cty stdlib. Here we allow
them to be used when constructing the value for a "literal" spec and in
the result expression for a "transform" spec.
This new spec type allows evaluating an arbitrary expression on the
result of a nested spec, for situations where the a value must be
transformed in some way.
This is essentially a CLI wrapper around the hcldec package, accepting a
decoding specification via a HCL-based language and using it to translate
input HCL files into JSON values while performing basic structural and
type validation of the input files.
