runtime: Allow to disable support for subgraphs

Use make NO_SUBGRAPHS=1
Saves 300 bytes of PROGMEM
Build: Default debug/subgraph feature to on always, explicitly disable
runtime: Fix inverted logic in debug checks

>-<, but at least automated tests caught them :)
runtime: Change HostCommunication/Transport API to a buffer with length

Shorter, pretties and potentially more efficient.
Shaves off 500 bytes PROGMEM
components: Tweak arduino-minimal to fit on devices like the Arduino Pro 168

Should juuust squeze in now if built with
make build MODEL=pro LIBRARY=arduino-minimal GRAPH=examples/null.fbp NO_DEBUG=1 NO_SUBGRAPHS=1

References #47