towerprime: extend Tower monad api to include Artifacts, Modules, Dependencies

also, system ast can calculate signal receivers just as it does channel receivers
towerprime-freertos: signals implementation filled in
towerprime-frontend: output Artifacts
towerprime-examples: new STM32F4 interrupt app doesn't quite build yet

need to write a makefile for this guy, and figure out how to delegate
appropriate isrs to the OS

I probably took too much responsibility out of the OS in the Signalable
class...