Imperative shell of the kernel.

Shell modules may modify the global context in a controlled and isolated way. This makes them easier to reason about than undisciplined global variables. They define a clear API that is exposed in kernel.api and used by the integration tests.

The data structures manipulated by shell modules are defined in the functional kernel.core.

Shell modules include kernel.shell.context, kernel.shell.site, kernel.shell.client and kernel.shell.server.