Revert "Add back thenable coercions weak map. Closes #62."

This reverts commit bbc5b5caf2abb2df42354f1d2ed3bb4122422af1. As per #79, the thenable coercions weak map is, at least in its current form, not a good idea.

This also fixes #82, since the realm is no longer needed.
Add missing completion values. Fixes #81.