Using GADT's to maintain invariant in GHC libraries

I rewrote compiler/utils/OrdList.lhs using a GADT to preserve some invariants stated only in comments — an excellent chance for ghc to eat its own proverbial dog food I feel :)

The type signatures remain the same so it should be a drop-in replacement but it does use some GHC features that are too recent (closed type families in particular, the IsList instance can be removed) making it ineligible for inclusion I believe.

hvr on #ghc suggested that I submit a patch anyway and that it could be implemented at a later date (they said ghc must be able to be compiled by the last two previous major versions), if this is useful then I will translate Bag.hls which has the same invariants but more complicated functions.

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Trac field	Value
Version	7.9
Type	Task
TypeOfFailure	OtherFailure
Priority	lowest
Resolution	Unresolved
Component	Compiler
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