Add "queuing" FSM

[tkem]

As stated in #5, process_event() must not be called recursively. Since this is a common pattern, especially with multiple FSMs passing events to each other, a "queuing" FSM type should be added.

[tkem]

Things to consider:

• type of queue/container
• priorities?
• exception handling with pending events

[tkem]

Type of queue container should be an additional template argument. No default should be provided for compatibility with freestanding implementations.

[tkem]

Investigate whether this could be an additional processing_lock template parameter; then we'd have something like

• no_lock for NDEBUG and -fno-exceptions
• throwing_lock as default if exceptions are supported
• queuing_lock to queue/postpone new events during event processing