Better support for objects that don't have a primary key called :id

I think we could add a method to `IModel` called `primary-key` or something like that that could be used to get field(s) that should be used as the primary key (default implementation would return `:id`). It could accept either a keyword (for a single-column key) or a sequence of keywords for multiple-column keys.

Start by locating the IModel definition and the code paths that assume a primary key named :id. Review how model fields are used for lookup and persistence, then determine the tests needed for single-key and composite-key models; done means both forms work while the default remains :id.

Written by the indexing model from the issue text.