links#find should now be resolved with store.findLinked

Source#findLink now fetches IDs, while findLinked fetches records.
Moved attr, hasMany, and hasOne to fields dir

This is in preparation of introducing `key`.
Compatible with new flexible key schemas in Orbit

* There is no longer a single `idField` and `remoteIdField` per schema
* Every model can have a unique primary key and any number of secondary keys
* Keys are defined with the new `key` helper
* The primary key value is always mapped to `primaryId` on each model
* If no primary key is defined for a model, a UUID-based `id` field will be defined automatically