Make keys the second argument to validations

I've revamped verily to use the notation

    :validation <keys> [args] [msg]

instead of

    :validation [args] <keys> [msg]

That's for three reasons:
Ensure key-keyword is distinct from msg-keyword when i18n-support is
implemented:

    [:validation :key <args> :my.custom.msg]

The other reason is that it makes the implementation of the
type-validation/custom-validation merge in formative slightly simpler.

Also, this way it's more consistent, I think. What do you think?