Consider supporting foreign promises

[clue]

Originally suggested by @jsor in #18 (comment).

Likely depends on reactphp/promise#45

[jsor]

v2.4.0 is released with support for foreign thenables.

[clue]

I agree that it makes more sense to keep relying on promises instead of duplicating similar logic here 👍

I'll assume this is resolved and will close this for now, please feel free to report back otherwise 👍