Implement RFC#28: Add PartialOrd::partial_cmp

I ended up altering the semantics of Json's PartialOrd implementation.
It used to be the case that Null < Null, but I can't think of any reason
for an ordering other than the default one so I just switched it over to
using the derived implementation.

# Note
This isn't ready to merge yet since libcore tests are broken as you end up with 2 versions of `Option`. The rest should be reviewable though.

RFC: 0028-partial-cmp