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WIP: Make mapCoercion zap Refl coercions

Simon Peyton Jones requested to merge wip/T17223-map-coercion into master

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?

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