ENH: add Timestamp.to_datetime64
This PR adds a `Timestamp.to_datetime64()` method to complement the `Timedelta.to_timedelta64()` method I added in #8884. It is a continuation of the aborted #8916.

Arguably, there should also be the alias `Timestamp.values` to complement the series property but I haven't added that yet.

hmm, its called `Timestamp.value`; you think `.values` should also be there? why? (its not a big deal, but curious why)
 `Timstamp.value` is an integer; `.values` would be a `np.datetime64` object, like the datetime64 `.values` on arrays.
 hmm that could be useful
 Any comments on this PR? (Tests are green)
 trivial comment. looks good otherwise. you are picking up pieces from those other pr's yes?
 @jreback By other PRs do you mean #8916? Yes, probably, eventually. Not entirely sure it's possible though given numpy datetime64 limitations :).

I wrote this one because I need it for the next PR I'm submitting tonight.
 @shoyer yep. you had some good stuff in there IIRC.
