This allows us to more clearly define interfaces between modules, and have the
linker help us ensure that module boundaries are respected.
The onion/tcp/net_crypto layer is a bit too large. This is due to a cyclic
dependency (onion -> net_crypto -> TCP -> onion). We may or may not want to
break that cycle in the future to allow the onion library to exist on its own
without net_crypto.
Also, fix the hstox build that was taking half an hour. It now takes 5 minutes.
Also, perform distcheck on travis to ensure that make dist works. It's not
actually failing the build at the moment due to broken tests.
This is in preparation for having multiple types of build. One of the future
builds will be a hstox build, another may be frama-c or some other static
analyser. It makes sense to split these up into multiple builds, because each of
them can take a while, and running them in parallel will speed things up. Also,
the hstox test coverage should be reported separately from the toxcore auto_test
coverage.
