strengthen binding between `Event.type` and `Event.target`

[cfm]

In the v2 Journalist API, events are explicitly typed as a 2-tuple of:

1. Event.type
2. Event.data (optional)

It occurs to me that they are implicitly typed further as a 3-tuple of (1) and (2) plus:

3. Event.target

Although (2) is optional, it's strong: An event will be rejected if its data has the wrong schema for its type.

By contrast, although (3) is required, it's weak: A handler will attempt to process an event with a valid target of the wrong schema, for example an ItemDeleted event with a SourceTarget instead of an ItemTarget. The handler will fail correctly, but it will fail while handling the event rather than while validating it.

I see three options:

1. Do nothing. Events with invalid targets will continue to fail during handling rather than validation.
2. Define EVENT_TARGET_TYPES just like EVENT_DATA_TYPES. Reject events with invalid targets during validation.
3. Define a parameterized decorator. Let individual handler methods reject events with unexpected targets, e.g.:
   • @expect(SourceTarget)
   • @expect(ItemTarget)
   • @expect(ItemTarget, SourceTarget)

[cfm]

Discussed with @legoktm: To keep this as simple as possible, option (2) it is.