Per suggestions from Simon M:

* Changed GHC.checkModule so that it doesn't call depanal.
* Changed GHC.checkModule to optionally return Core bindings
as a component of the CheckedModule that it returns (and 
resulting changes to HscMain.hscFileCheck).
* As a result, simplified GHC.compileToCore and changed it
to load the given file so that the caller doesn't have to.