Move where -> let binding transformation to the desugaring step

[bergmark]

This is done in Fay.Compiler.Decl:

  PatBind srcloc (PVar _ ident) Nothing (UnGuardedRhs _ rhs) (Just bdecls) ->
    compileUnguardedRhs toplevel srcloc ident (Let S.noI bdecls rhs)

This should also not throw away the srcloc, it can be kept in the Let.

Move this to Desugar and add and throw an error on this case in the Decl module.