- isa/can validation in XS allows this:

 { a => { isa => \%hash } }

and tries to see if a->isa("HASH(0x1581)").  Instead, we should die if
given a non-array reference.  Same for can, etc.  See how this is
handled in callbacks for example.


- add a transformation callback feature, to be called before
validating a value, which can be used to normalize values
