split boot kernel and stdlib

The suggestion is to break boot into two parts the better to support tool makers who just want the boot kernel without the builtins and utils. A `boot.kernel` project would contain just `boot.core`, `boot.pod`, `boot.aether`, and whatever else is needed to make a minimal kernel for developers. Then `boot.stdlib` would contain `boot.task.built-in` and `boot.utils`, although you could make the argument that `boot.utils` should be in the kernel, or in a separate project. You could package both the kernel and the stdlib in a standard `boot` package for users, but developers could still use just `boot.kernel`.