Deprecate Module.io and BlackBox.io (#1550)

* Deprecate Module.io and BlackBox.io

This is a step toward unification of Module with MultiIOModule.
The future of BlackBox is a bit less clear, but its existing API can be
maintained without the io virtual method.

The trickier API to maintain is auto-IO wrapping for compatibility
Modules and BlackBoxes. This will probably require reflection to support
once the io virtual method is removed.

* Improve deprecation warning for io