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	Report top-level implicit parameter errors more nicely
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Consider
    module Main where

	main = let ?x = 5 in print foo

	foo = woggle 3

	woggle :: (?x :: Int) => Int -> Int
	woggle y = ?x + y

GHC's current rules say that 'foo' is monomorphic, so we get
	foo :: Int
but we also get an unbound top-level constraint (?x::Int).  GHC 6.2 emits a
message like:
     Unbound implicit parameter (?x::Int)
     arising from use of `woggle' at ...

The point is that THERE IS NO WAY FOR THIS CONSTRAINT TO GET BOUND,
because we don't have a top-level binding form for implicit parameters.
So it's stupid for 'foo' to be monomorphic.

This commit improves matters by giving a much nicer error message:

     Implicit parameters escape from the monomorphic top-level binding(s) of `foo':
       ?x::Int arising from use of `woggle' at tcfail130.hs:10:6-11
     Probably fix: add type signatures for the top-level binding(s)
     When generalising the type(s) for `foo'