Implement fmap/coerce rules

NOTE: This is not tested yet!

Implement fmap/coerce rules for Map, Sequence, and Tree. One concern:
unfortunately, implementing the RULES requires importing Data.Coerce,
which forces the LANGUAGE to be turned from Safe to Trustworthy.  This
is rather sad. An alternative would be to do this in another module, but
orphan rules are not so lovely either.
I don’t think much changed with regard to RULES and coercion since 7.8, and even if the rule does not fire it is not a problem, I think.