Merge core into sysand

[tilowiklundSensmetry]

We may want to make command line argument parsing available to language bindings, for example in order to implement #12 .

Currently, this would require either moving part of command line handling into core, or to have language bindings depend on sysand instead of core.

In either case, the reason for keeping a separate core package becomes less clear. We should therefore consider simply merging the two into one crate (and hide the actual CLI binary behind a feature flag).

[andrius-puksta-sensmetry]

Why use feature flags for binary? To not compile/have clap as a dependency in library?