Properly support "enabled_when_ready" attribute for methods

As mentioned in this comment on Discussion #605, there is some support for enabled_when_ready as a method attribute, but it is not parsed for BH (only BSV) and the use is not checked in the parent module. And the feature is not tested or documented. Implementation seems straightforward, although some high-level thought may be needed, to decide if we want to reuse the existing encoding at synthesis boundaries or extended it.