Simplify SimplCont, plus some other small changes to the Simplifier

Simplify SimplCont, plus some other small changes to the Simplifier

The main change in this patch is this:

  * The Stop constructor of SimplCont no longer contains the OutType
    of the whole continuation.  This is a nice simplification in
    lots of places where we build a Stop continuation.  For example,
    rebuildCall no longer needs to maintain the type of the function.

  * Similarly StrictArg no longer needs an OutType

  * The consequential complication is that contResultType (not called
    much) needs to be given the type of the thing in the middle.  No
    big deal.

  * Lots of other small knock-on effects

Other changes in here

  * simplLazyBind does do the type-abstraction thing if there's
    a lambda inside.  See comments in simplLazyBind

  * simplLazyBind reduces simplifier iterations by keeping
    unfolding information for stuff for which type abstraction is
    done (see add_poly_bind)

All of this came up when implementing System IF, but seems worth applying
to the HEAD