Follow mtl upstream

As per http://projects.haskell.org/pipermail/haskell-platform/2012-March/001760.html we should be tracking https://github.com/ekmett/mtl.git (which may mean asking upstream to merge our patches, if we have any) and will also need to track http://code.haskell.org/\~ross/transformers

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Trac field	Value
Version	7.4.1
Type	Task
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Build System
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