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This is just a tidy-up.  Previously we were calling occurAnalyse
twice on each LHS which was silly and a bit unclean too.  

This patch should have no overall effect, though.

Max has some more heuristics to add, and is meanwhile worried
that having SAT on by default will make some highly-tuned array
programs worse.  So it's off by default.  

Use -fstatic-argument-transformation to switch it on.

GHC allows a non-recursive let for type varaibles
	let a = TYPE ty in ...
But the free-variable finder had not caught up with this
fact. This patch catches up.

A type let shouldn't really occur in SetLevels, but if it does,
this patch makes sure it is left alone.

I'm not sure I covered all the ways of throwing errors in the code.
Some functions throw ProgramErrors, some error messages.  It's still
quite a mess, but we're getting closer.  The missing cases are mostly
errors that cannot be fixed by the API client either or are a result
of wrong usage, so are in any case fatal.

One function, 'getModuleInfo', still returns a 'Maybe', but the
documentation suggests it should always succeed.  So I may change that
soon.

The spit-up of of 'checkModule' has pros and cons.  The various forms
of 'checkModule*' now become:

 checkAndLoadModule ms False ~~>
    loadModule =<< typecheckModule =<< parseModule (ms_mod_name ms)

 checkAndLoadModule ms True ~~>
   loadModule =<< desugarModule =<< typecheckModule =<< parseModule (ms_mod_name ms)

 checkModule mn False ~~>
   typecheckModule =<< parseModule mn

 checkModule mn True ~~>
   desugarModule =<< typecheckModule =<< parseModule mn

The old APIs cannot easily be provided, since the result type would be
different depending on the second argument.  However, a more
convenient API can be modelled on top of these four functions
({parse,typecheck,desugar,load}Module).

This monad will be required by most public API calls.

printing them.

This monad provides variants of 'catch', 'bracket', and 'finally', so
exceptions can be handled in monads that wrap IO.  The latter two
methods need to be part of the class definition, because GHC uses
'block' and 'unblock' which are used in the definition of those two
methods for the IO monad.  A perhaps better class interface would
consist of 'gcatch', 'gblock', and 'gunblock' and let the latter two
default to 'id' like is done for non-GHC implementations of 'bracket'
and 'finally'.

This suspicious function had just one call, in BuildTyCl.mkNewTyConRhs.
I've done it another way now, which is tidier.

I did the wrong checkSigTyVars, which (happily) triggered an ASSERT
failure.  This should fix it.