Executors & Schedulers facilities.

Bulkhead pattern: limiter of concurrent executions.

A core.async analogous implementation of channels that uses promises
instead of callbacks for all operations and are intended to be used
as-is (using blocking operations) in go-blocks backed by virtual
threads.

There are no macro transformations, go blocks are just alias for the
`promesa.core/vthread` macro that launches an virtual thread.

This code is based on the same ideas as core.async but the
implementation is written from scratch, for make it more
simplier (and smaller, because it does not intend to solve all the
corner cases that core.async does right now).

This code is implemented in CLJS for make available the channel
abstraction to the CLJS, but the main use case for this ns is
targeted to the JVM, where you will be able to take advantage of
virtual threads and seamless blocking operations on channels.

**EXPERIMENTAL API**

Concurrency limiter: Semaphore

A generic promise abstraction and related protocols.