Check whether signals escape their when scopes (#1518)

* Include and check when scoping as part of reg/mem/wire/node bindings

* Allow outdated 'when' behavior of CHIRRTL memory ports with enables

* Extend cross-module / when-visibility checks to all data refs

* Fixes #1512
* Cannot be checked if outside a module context
* E.g. delayed evaluation of printf / assert args

* Add basic test cases for cross-module refs / signals escaping when scopes

* Remove illegal cross-module references from existing tests