Major improvement to SpecConstr

Major improvement to SpecConstr

This patch improves the SpecConstr pass, by
  a) making it work with join points
  b) making it generate specialisations transitively

As part of it, SpecConstr now carries a substitution with it, which
runs over the whole program as it goes.  This turned out to be
a big win; simplified the implementation quite a bit.

I have *disabled* the specialisation on lambdas; it's pretty fragile,
and sometimes generates more and more specialisations. Something to
come back to, perhaps.

I rejigged the flag-handling a bit.  Now the specification of passes
in DynFlags is a bit nicer; see
	- optLevelFlags top-level data structure
	- runWhen function
	- CoreDoPasses constructor

There are now command-line flags
	-fspec-constr
	-fliberate-case
	-fspec-threshold=N
which do the obvious thing.  -O2 switches on both spec-constr and liberate-case.
You can use -fno-liberate-case, -fno-spec-constr after -O2 to switch them off again.

The spec-threshold applies to both these transformations; default value 200 for now.