First draft IVar-Future-Promise-Probe unification

Not ready for merge. Opening discussion. Compatibility layer is totally missing for now.
Actually, this method would be better to be placed elsewhere since it's not Future related. It just allows to post (hence the same name of the method) a task to one of the global pools.

Original `#execute` relates most closely to `#future` method now.

My main motivation for naming was to make this new implementation as coherent and surprise-less as possible, since this brings so many changes I did not thought much about original names.