improve usability of machine-runner API

[rkuhn]

The purpose of the machine-runner library is twofold: it should provide a safe and intuitive API for writing, running, and inspecting state machines, and it should allow static analysis of a set of such state machines plus a swarm protocol to ensure protocol correctness and adherence.

For the first aspect we need the following:

• type-safe construction of the initial state (for production code) as well as all other states (for unit tests)
• type-safe inspection of an unknown runtime state (“downcasting” and access to properties), e.g. in a UI or a node.js application
• type-safe invocation of commands if the runtime state is of a particular name
• type-safe formulation of events emitted from commands and events consumed in handlers as per the overall protocol
• a generic handler function to be invoked upon reception of an unexpected event

For the second aspect we need the following:

• identification of initial state
• enumerate all commands including their name, argument type list, and resulting event list
• enumerate all event handlers including argument type list and name of the resulting state

These can be captured during unit tests (i.e. by reifying states, events, handler, and commands as objects) or analysed using TypeScript as a library; in either case, the captured machine specification needs to be formatted in JSON and passed to an external type-checking program for swarm protocols (currently implemented in Haskell).

[rkuhn]

moved discarded event handler to #25

The rest is fixed by #7.