Constrain layout messages to be members of a Message class

Using Typeables as the only constraint on layout messages is a bit
scary, as a user can send arbitrary values to layoutMsg, whether they
make sense or not: there's basically no type feedback on the values you
supply to layoutMsg.

Folloing Simon Marlow's dynamically extensible exceptions paper, we use
an existential type, and a Message type class, to constrain valid
arguments to layoutMsg to be valid members of Message.

That is, a user writes some data type for messages their layout
algorithm accepts:

  data MyLayoutEvent

darcs-hash:20070504081649-9c5c1-954b406e8c21c2ca4428960e4fc1f9ffb17fb296.gz

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