Working prototype for wider hooks/lifecycle support in arc

[filmaj]

See the plugins branch in this repo for work in progress. This is to be paired with copper's fork of architect (plugins branch) (which itself pulls in various forks of the arc sub-packages).

The intention is to implement a prototype as a showcase for solving architect/architect#1051.

[filmaj]

Plugins support is live in arc with 8.5.0-RC.2 and newer, and the plugins branch for this repo is now merged in! Closing as complete!