We need to be able to set hooks from the outside. This takes place in two API sets: the implementation hooking API, and the convenience fluent API.

The implementation hooking API is two calls which have object args and return boolean for success:

1. set_hook({ ... })
2. remove_hook({ ... })

The convenience fluent API is a call for each hook and unhook type, take positional args, and return the machine so that they can chain. As a few examples, non-exhaustively:

1. hook('a', 'b', handler)
2. hook_action('a', 'b', 'demo', handler)
3. unhook('a', 'b')

API calls are described at #660.

Fluent calls are described at #699.