CLOS-like extensible instance initialization

[fare]

In CLOS, users can modularly declare how to initialize their slots based on keywords that will be used by make-instance, initialize-instance, reinitialize-instance, shared-initialize, and more.

Duncan Mak expresses the desire to use such a feature to port DEUCE / DUIM from Dylan. That would be a good opportunity to add these features.

See the CLHS: https://www.lispworks.com/documentation/HyperSpec/Body/07_.htm