WIP: Make mapCoercion zap Refl coercions
This is work in progress on issue #17223.
The main idea is for mapCoercion_maybe
to return a MCoercion
that, in the Refl case, does not have to zonk (or even look at)
the type. Insetad, the cast or wrapper simply disappears.
There is a lack of comments at the moment.
The SOURCE import of Coercion in Type.hs seems to import too much. Mabye moving mapCoercion to Coercion would help?