- 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.


- make it possible to explicitly not use the XS version in a non-hacky
way.


- is there something better than "N/A" that could be used when
validation is being done outside a sub?


- add an excludes param to the spec as the opposite of depends,
suggested by Diab Jerius (see rt.cpan.org).