Systems API

[kvark]

This is a kick-off issue to start thinking about systems/processors, and how they can process large amounts of data in parallel. Includes, but is not limited to, the following topics:

• do we need systems at all? calx-ecs doesn't have them, for example
• should we limit the scope of systems to ECS emulation (Emulate ECS #28) only?
• possible integration with shred
• support for internal parallelism with rayon

At last, for those who enjoy some reading, here is the original discussion of the time specs was born: amethyst/amethyst#10. It has a different scope, but some ideas can be seen in the new light of expanded capabilities that CGS provides.