Migrate from futures

[caspiano]

Strip dependency on future.cr, potentially replace with promise.cr

[watzon]

Just gonna drop this as a suggestion. I created https://github.com/protoncr/async which takes the idea of futures that exist in future.cr and extends them. Also provides async/await macros which basically just wrap the body of your function in an Async::Future and executes it immediately when called.

It also has some methods from JavaScript promises like Future.all, Future.race, and Future.any.