API for events and hooks

[lionel-]

• task callbacks
• hooks and events
• deferred expressions (exits)

Needs to wrap 4 actions: append, prepend, replace, get.

Appending and prepending might be handled with push_ prefixed verbs and prepend = FALSE argument. Replacing would be dealt with poke_ verbs, and we'd have peek_ verbs to retrieve the current hooks.

[hadley]

Note that some (many?) don't offer a get option.

peek_ and poke_ don't feel very "event"-y to me. I'd prefer something like on_ or when_

I think we could make append/prepend/replace an argument to a "set" function.

[hadley]

Note that finalisers (e.g. reg.finalizer()) also fall into this category (if you're not already considering as part of deferred expressions)