Finally, since normally you need the content from `._context.Context`
and surroundings in order to effectively grok `Portal.open_context()`
anyways, might as well move the impl to the ctx module as
`open_context_from_portal()` and just bind it on the `Portal` class def.
Associated/required tweaks:
- avoid circ import on `.devx` by only import
`.maybe_wait_for_debugger()` when debug mode is set.
- drop `async_generator` usage, not sure why this hadn't already been
changed to `contextlib`?
- use `@acm` alias throughout `._portal`
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