Single and Completable converters

[akarnokd]

This PR adds direct converters to and from Completable and Single.

[ldaley]

@benjchristensen are you still the owner here? If not, who should be considered the new owner?

[benjchristensen]

I'm not actively engaged and don't have the ability to commit appropriate attention. @stevegury, @stealthcode, or @akarnokd can do anything needed.

[stevegury]

👍
@akarnokd Do you have member acess on this repo (can you merge?)

[akarnokd]

Yes.