Decouple `context` from `runtime<N>`

[kammce]

We may want to provide a number of options for allocating and managing context types, but making them impossible to use without the runtime<N> type seems a bit too restrictive. The benefit of the runtime<N> is that it technically reduces the memory cost of a context by sharing the core information across N context types. Runtime contains the transition_handler that is accessible via the runtime_base type.

[kammce]

Adding the above dependencies as these are needed to actually decouple runtime from context. Specifically, since runtime will no longer store the transition_handler we can either have a copy everywhere or a strong_ptr to the soon to be interface. The strong_ptr is the sane route in my mind.

[kammce]

Closed by #21