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	Keep-alive set and Template Haskell quotes
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a) Template Haskell quotes should be able to mention top-leve
   things without resorting to lifting.  Example

	module Foo( foo ) where
	  f x = x
	  foo = [| f 4 |]

   Here the reference to 'f' is ok; no need to 'lift' it.
   The relevant changes are in TcExpr.tcId

b) However, we must take care not to discard the binding for f,
   so we add it to the 'keep-alive' set for the module.  I've
   now made this into (another) mutable bucket, tcg_keep, 
   in the TcGblEnv

c) That in turn led me to look at the handling of orphan rules;
   as a result I made IdCoreRule into its own data type, which
   has simle but non-local ramifications