Support bidirectional exec/operation

[J-Rojas]

It would be useful to support a bidirectional promise between parent and worker and also worker and parent.

For example, the worker may need to execute and wait for an operation only available to the main thread, so having exec/operation work within the context of the main thread should be supported.

[kwolfy]

It looks like good idea, will happy to see PRs

[J-Rojas]

Hi, I've posted a PR for this issue.