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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.)

It makes *sense* for a foreign import to have a zero-sized return, thus

  foreign import ccall foo :: Int -> State# RealWorld

but it's not clear that it's useful, and it requires some back-end (a
Hint for void types) to make it go right through. It's not clear that
we really want this facility, so rather than fixing the code
generator, I'm just making the construct illegal for now.

Trac #1581 was doing too much filtering; it even filtered out intances 
defined in this very module!  The new rule shows more instances, but
hopefully not to many.

Furthermore I have moved the filtering out of TcRnDriver (where it does
not belong) to InteractiveEval. And I've added a note to the documentation.

So we will inherit default values for any new flags that get added,
so things should not break if new flags are added.

Defaulting makes compilation of multiple modules more complicated (re: #1405)
Although it was all locally within functions, not because of the module
monomorphism-restriction... but it's better to be clear what's meant, anyway.
I changed some that were defaulting to Integer, to explicit Int, where Int
seemed appropriate rather than Integer.

Print the list of strings that are accepted in a LANGUAGE pragma,
or as a -XFoo flag. (No can be prepended to any of the strings).

-fparr did exist before, it just wasn't documented.

Now we only allow -XFooBar syntax, not alternate case, hyphens or -f.
There are some deprecated -f flags accordingly.

Adaptated from code from Sigbjorn Finne

Fixes read002 on x86_64 (and maybe others).  The stg_ap_f_info info
table had the wrong liveness on it, because the float had been treated
as having zero size.

A Name used to have a Parent, but no longer has.  When we want to 
print info about data type T and data constructor MkT, the info about
T already describes MkT so we want to discard the latter.  Now that
Names don't have a Parent, we must do that in a different way,
using implicitTyThings

Test is ghci011