Use core's AssertParamIsClone and AssertParamIsCopy traits

[CohenArthur]

These traits are used to produce better errors when expanding derive(Clone) macros, but they are not lang items. Hence we cannot refer to them using a lang-item path, and instead need to use a full-blown core path (::core::clone::AssertParamIsCopy). This is problematic for us as we cannot yet compile or distribute core, and thus we do not always link user code to it.

Once we have figured out how to distribute core, we'll be able to make use of these assertions.