KV store: core storage follow-ups

[nrc]

• Panic on non-unique foreign keys in indexes
• Singletons are attached to a specific store rather than being free
• Owners are specified as part of the schema rather than on first use
• Helper type of a KvStore + Owner (similar to Table but for accessing singletons)
• Check indexes for disjointness (debug assert)
• Possibly make singletons fields of the store rather than use a HashMap
• kv: remove Clone requirement for iterators, declutter lifetimes #209