Add diff method and comparisons to Asn1TimeRef

This implements a `diff` method on `Asn1TimeRef` using `ASN1_TIME_diff`
and uses this new method to implement combinations of `PartialEq` and
`PartialOrd` for `Asn1Time` and `Asn1TimeRef`.

This is mostly just a rework of the earlier work done by @illegalprime
in his PR #673 and credit should go to him.