Before this change we used to exec() various binaries to find out memory
information.

While this worked, it was awfully slow.

And if somebody would want to compute how many percent of available memory all
PIDs on the system uses, that would take almost ten seconds on my laptop with
the previous implementation.

This implementation fares a lot better, and is smaller.

This has the benefit of allowing us to also grab the cached memory so
that we can get an "available" measurement

Currently, test should be failed with NotImplementedError or something.