Improve implementation of unSubCo_maybe.
This is the result of an email conversation (off list) with Conal Elliott, who needed a stronger unSubCo_maybe. This commit adds cases to upgrade the role of a coercion when recursion is necessary to do say (for example, for a use of TransCo). As a side effect, more coercion optimizations are now possible. This was not done previously because unSubCo_maybe was used only during coercion optimization, and the recursive cases looked to be unlikely. However, adding them can cause no harm. unSubCo_maybe is now also exported from Coercion, for use cases like Conal's.
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