Vendor deprecated/unstable traits from std::num

This commit brings in a load of unstable and/or deprecated traits from
the `std::num` module. These traits provide for some degree of generic
programming over numeric types. They are not stable in `std` mostly
because we want more time to iterate on their design. Moving them to the
`num` crate allows existing code to keep using this functionality as we
do so.

Closes serde-rs#74