Implement fmap/coerce rules for GHC

[treeowl]

GHC.Base decrees

{-# RULES "map/coerce" map coerce = coerce #-}

This rule applies equally well to any legitimate Functor instance (any Functor instance that obeys the functor laws), including various ones in containers.

[foxik]

Patches welcomed :-)

[treeowl]

I'll try. I got a bit bogged down implementing this for Array today. I also keep forgetting how to get upstream repos properly set up and synced with git, but I'll get that worked out for containers shortly, so I can do this properly.

[foxik]

Note for myself -- #71 (comment) still needs to get sorted out.

[treeowl]

All are done except for SCC in Graph, and that's not too important because it should already be able to take advantage of the rules for lists.