Helper functions for slug processing

In support of #151, the following transforming library for slugs should be added:

1. getSlugFromMetadata(__rerum:Object) -> validSlug:String
   • extract "this-slug" from DB record property {__rerum: { slug : "this-slug" } }
   • throw invalid argument if not __rerum metadata
   • return empty "" if slug property empty or missing
2. assignSlug(slug:String, document:Object) -> document:Object
   • insert or overwrite "this-slug" to __rerum property for traditional save
   • empty/remove slug with empty ""
3. findBySlug(slug:String) async -> document:Object
   • RERUM lookup for slug match
   • 404 on a miss, 400 on a bad slug
4. transferSlug(toDocument:Object, fromDocument?:Object) async -> 200 <location>
   • RERUM upsert toDocument with "this-slug" and RERUM patch fromDocument with { __rerum.slug: "" }
   • A single transaction, revert on a failure
5. slugExists(slug:String) async -> Boolean
   • *findBySlug(slug) is not 40x
6. slugIsValid(slug:String) -> Boolean
   • slug and normalizeSlug(slug) match
7. normalizeSlug(userSlug:String) -> validSlug:String
   • "This, That & the Other! Various Outré Considerations" -> "this-that-the-other-various-outre-considerations"
8. buildSluggedId(validSlug:String) -> atIdUrl:Url
   • "this-slug" -> "https://store.rerum.io/v1/id/this-slug"

Only "normalizeSlug" takes user input.

Comienza leyendo el issue relacionado #151 para entender el contexto de procesamiento de slugs solicitado. Después, define los contratos de los helpers indicados y sus comportamientos de validación, búsqueda, normalización y transferencia según este issue. El trabajo estará terminado cuando todos los helpers especificados admitan los casos documentados de éxito y error, incluido el rollback de la transferencia de slugs.