First-class-ish support for slugs

[stephenh]

Slugs in URLs are similar to tagged ids, but incorporate part of the entity's data directly, i.e. Author firstName=bob might have an Author slug=bob-123, which looks tagged-ish, but isn't actually immutable (users can change the firstname, username, etc).

There are Rails gems that incorporate slugs into ActiveRecord, i.e. friendly_ids, so it'd be interesting to see if we could do something similar with Joist.

This would very likely just be a use case that was supported by a plugin, and not a core feature, but one could imagine a query rewriting plugin that would:

• See a where = { author: "someString" } clause for an em.find/etc
• Pattern match the some string has "probably/definitely a slug
• Rewrite the query AST to be where = { author: { slugs: { slug: "someString" } } }, i.e. a join into the slugs table that keeps track of the 1-N slugs that have been created for the given Author

If we were super-fancy, the domain.id might start returning slugs, which at least for Joist would be fine because we've moved basically all of the internals over to idTagged to support maybe/maybe-not tagged ids.