Rework implicit async core

[AdvancedAntiSkid]

• Move context mapping outside of Future. Introduce an interface, and maybe add the context system as a default implementation.

• Introduce fibers via a work-stealing scheduler, to improve pre-jdk-loom os threads, by suspending and rescheduling tasks. Two possible approaches are either using a runtime agent to reinstrument fiber executable targets or using a maven/gradle plugin to manipulate the build process.