Functional core of the kernel.

Every core module only comprises pure functions that do not affect the global context. This simplifies reasoning about modules and their dependencies and eases testing.

Most modules correspond to a data structure in the global context that is only manipulated from the imperative kernel.shell.

Core modules include kernel.core.causal-dag, kernel.core.compound-operation, kernel.core.conflict-cache, kernel.core.conflict-relation, kernel.core.feature-model, kernel.core.garbage-collector, kernel.core.history-buffer, kernel.core.message, kernel.core.movic, kernel.core.primitive-operation, kernel.core.topological-sort and kernel.core.vector-clock.