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?