Small improvement to CorePrep

Small improvement to CorePrep

This change avoids unnecessary bindings. Example

     foo (let fn = \x.blah in
          in fn)

We were generating something stupid like

    let fn = \x.blah in
    let fn' = \eta. fn eta
    in foo fn

Now we don't.  The change is quite small.

Thanks to Ben for showing me an example of this happening.