registerCallback returning Any considered harmful

[thirtytwobits]

In the current design all of the places where we have Any registerCallback(Function&&) inhibit the ability of users to author custom executors and to integrate with various platforms. The primary issue is the method is required to move a node into an opaque container that is then moved into an unknown caller. There is no canonical key available for the implementation of registerCallback to retain. This prohibits filtering callbacks within execution stages based on type and writing event-driven rescheduling of callbacks by platform layers.

We need to modify this design to use a allocation pool instead of Any types so there is only even a single memory location containing a given callback within a process.