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I decided against adding parseSignedInteger since octal
and hex literals often have junk between the '-' and the
digits, but, compare to Util.readRational which does handle
signed numbers.  Also since Integers - mathematically and
in Haskell - can be negative, normally.

It doesn't work yet, but I'm pretty sure that's because the bindist is
broken rather than the installer is broken.

Not done by default yet, but useful when hacking on libraries.

Roman produced programs involving associated types that did not optimise well.
His programs were something like this:

  data family T a
  data instance T Int = MkT Bool Char

  bar :: T Int -> Int
  bar t = t `seq` loop 0
	where
	  loop = ...

You'd think that the `seq` should unbox 't' outside the loop, since
a (T Int) is just a MkT pair.  

The most robust way to make this happen is for the simplifier to understand
a bit about type-family instances.   See 
	Note [Improving seq]
in Simplify.lhs.  We use FamInstEnv.topNormaliseType to do the interesting
work.

To make this happen I did a bit of refactoring to the simplifier
monad.

I'd previously done a very similar transformation in LiberateCase, but it
was happening too late.  So this patch takes it out of LiberateCase as
well as adding it to Simplify.

Consder Trac #1265, which does this in GHCi:

	Prelude> let doit = fail "Code not written yet" :: ExpQ
	Prelude> $(doit)

Even though 'doit' is defined "in the same module", it's OK to use it in a splice
because it'll have been fully compiled to bytecode.  (Contrast the situation if
these two lines appeared in a single, compiled module.)

Hence we want to bind 'doit' at TH's "imported level" (TcRnTypes.impLevel).  
This used to happen because GHCi-bound Ids were in the *global* type
env (and hence at "imported level").  But since SimonM moved
GHCi-bound ids to the *local* type env (for good reasons) the above
program has been rejected.

This patch makes it work again.