BUG/ENH: cleanup for Timedelta arithmetic

Fixes #8813 
Fixes #5963
Fixes #5436

If the other argument has a dtype attribute, I assume that it is ndarray-like
and convert the `Timedelta` into a `np.timedelta64` object. Alternatively, we
could just return `NotImplemented` and let the other type handle it, but this
has the bonus of making `Timedelta` compatible with ndarrays.

I also added a `Timedelta.to_timedelta64()` method to the public API. I
couldn't find a listing for `Timedelta` in the API docs -- we should probably
add that, right?

Next up would be a similar treatment for `Timestamp`.

CC @immerrr
do we usually have message with NotImplemented? (Ideally would like to, not sure if its convention)