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	Allow implicit-parameter bindings anywhere that
		a normal binding group is allowed.
	------------------------------------------------

That is, you can have implicit parameters

	* in a let binding
	* in a where clause (but then you can't have non-implicit
	  ones as well)
	* in a let group in a list comprehension or monad do-notation

The implementation is simple: just add IPBinds to the allowable forms of HsBinds,
and remove the HsWith expression form altogether.   (It now comes in via the
HsLet form.)

It'a a nice generalisation really.  Needs a bit of documentation, which I'll do next.