py/obj: Fix comparison of float/complex NaN with itself.

IEEE floating point is specified such that a comparison of NaN with itself
returns false, and Python respects these semantics.  This patch makes uPy
also have these semantics.  The fix has a minor impact on the speed of the
object-equality fast-path, but that seems to be unavoidable and it's much
more important to have correct behaviour (especially in this case where
the wrong answer for nan==nan is silently returned).