split boot kernel and stdlib

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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.