[M2] Begin implementation of the 'app2' module.

[evanlinjin]

Context

The app2 module aims to improve on and replace the app module (eventually). For now, we should use app2 to implement direct dmsg connections to the skywire hypervisor.

Specifications: https://github.com/watercompany/skywire-specs/tree/master/proposals/evanlinjin-app2