Refactor reporters to be factory-created event listeners

[jason0x43]

Currently reporters are all implemented as classes based on a Reporter root class. Internally, however, they are simply event listeners. The reporter class structure was intended to provide some shared functionality for reporters, but it really ends up being overhead, and it obscures how the reporter system actually works.

Reimplement reporters as simple event listeners created by factory functions. Put any shared functionality in a utility module or modules.

See #1023 (comment).