core.async: Add a function to set thread executor

Question

See slack #babashka discussion https://clojurians.slack.com/archives/CLX41ASCS/p1677319666536479

The use case is setting a virtual thread pool executor with SCI.
In Clojure I could set clojure.core.async.impl.exec.threadpool/thread-pool-executor.

Doing the intern in SCI will not have the desired effect since the pre-compiled functions don't see the change.

Clojure.core agents also allow this via set-agent-send-executor! etc.

Comments

Also see this SO answer where the workaround of overriding the thread executor is a bit gnarly:

https://stackoverflow.com/a/38577871/6264