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	Several small but tiresome things shown up by Template Haskell
	--------------------------------------------------------------

1. Make the 'knot' in TcRnDriver much smaller; in fact move it to
   TcIfaceSig.tcInterfaceSigs.  Reasons
	a) much tidier
	b) avoids a loop in Template Haskell, when we try to run
	   an expression during type checking (when the knot is
	   not fully tied)

   See comments in TcIfaceSig

2. Stop typechecking if tcGroup fails.  Reason: otherwise tcLookup can
   fail in the next group.

3. Catch linking errors more gracefully when running a splice (in TcSplice)