Integrate FuDGE/Trueflame's declarative loader/messaging system.

[jpstewart]

Integrating the messaging system is relatively straightforward and can be done better within CommonLib than from outside as Trueflame does it (i.e. registering listeners can be made idempotent, and therefore traditional and declarative messaging can coexist).

The loader will have to be mutually exclusive to the traditional mechanism. In Trueflame this is opt-in via the Cmake integration, which clib can use too.