The main thing in this commit is to change StgAlts so that
it carries a TyCon, and not a Type.  Furthermore, the TyCon
is derived from the alternatives, so it should have its
constructors etc, even if there's a module loop involved, so that
some versions of the TyCon don't have the constructors visible.

There's a comment in StgSyn.lhs, with the type decl for StgAlts


Also: a start on hscExpr in HscMain.