Make SpecConstr specialise for constant arguments again

Consider
  lvl = Just True

  foo :: Maybe Bool -> Int -> Int
  foo (Just True) i = i
  foo _           i = foo lvl i

SpecConstr should specialise foo, but it wasn't doing so (spotted
by Roman).

Reason: lvl's unfolding wasn't in the cloned version of lvl.
Solution: extend the value environment to record top-level bindings too

At the same time I made it work if 'lvl' is a lambda, in which case it
is again worth specialisg.  This meant renaming ConEnv to ValueEnv,
and adding a case for 'LambdaVal'.

(To make specialisation on lambdas work properly, we have to do lambda
lifting as well, but this gets part of the way, and fixes a bug too.)

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