Replace kernel parameter of Command methods with injection

ProBKernel is now a singleton (it's very unlikely that we'll ever need
to run multiple kernels in a single process, and if we do, each would
have its own injector anyway). This allows injecting it into commands
when they need it. This makes more sense than using a parameter,
because most commands don't need access to the kernel object.

We have to use an Injector and inject the kernel only when it's
actually used. We can't inject the kernel as a constructor parameter,
because the ProBKernel constructor injects all the commands, which
would cause a circular dependency.