runtime: Fix compile errors in Linux backend

Did not get updated when IO API changed in
9c6f97ce3d0493fdd16264c4ef2414b4429772b1
runtime: Change HostCommunication/Transport API to a buffer with length

Shorter, pretties and potentially more efficient.
Shaves off 500 bytes PROGMEM
runtime: Allow to disable support for subgraphs

Use make NO_SUBGRAPHS=1
Saves 300 bytes of 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