Add support for user hooks

#88 has added a preliminary hook framework for module events. This needs to be extended to handle *user hooks*, i.e. module events that can be hooked into from *other* modules, in analogy with [user hooks for package events](https://stat.ethz.ch/R-manual/R-devel/library/base/html/userhooks.html).

Start by reading issue #88 and its preliminary hook framework, then compare it with R's user hooks for package events in the linked documentation. Define how module events can be registered or invoked from other modules, and consider the work done when cross-module user hooks behave analogously to package user hooks.

Written by the indexing model from the issue text.