data transformation, via annotations produced by the "format" keyword (and possibly others)

[karenetheridge]

Add a mode which returns a transformed copy of the data that was just validated, using annotations from the result as a guide of what to do.

Requires awareness of various formats e.g. "int64" which act on strings but indicate the result should be transformed into an integer (likely a Math::BigInt object).

Can tie into the upcoming format registry, which will allow providing an additional subref which performs the transformation.

[karenetheridge]

This is similar to #96, where we produce a list of json pointers mapping to new values to populate into the instance data.

So perhaps the int64 format should produce a mapping of locations to Math::BigInt objects? or perhaps map subrefs that do the same.