Better abstraction of reference store

[jstolarek]

Reference store is an IntMap, with store labels being an Int. There are some helper functions that work on a store (deref, storeInsert, insertStoreHole). This is all a bit messy. StoreLabel should be a newtype, not a type synonym, Store should be a record with a map and reference counter (currently handled by Eval monad) and it should be completely abstract.

TODO

• [x] StoreLabel a newtype
• [x] StoreLabel instance of Valuable
• [x] StoreLabels a set
• [x] Store a pair of store and reference count
• [x] Store abstraction used in evaluation and repl monads.

[jstolarek]

Also, if StoreLabel is a newtype we could make it into an instance of Valuable, which would be a nice abstraction. At the moment we have to explicitly wrap it in VStoreLoc.

[jstolarek]

Also, StoreLabels should be a set.