TR should provide a Dictionary type

[lexi-lambda]

It would be nice if TR provided a built-in Dictionary type. At its simplest, it would just be (U (HashTable a b) (Listof (Pairof a b)), but vectors are also technically dictionaries.

Does TR's type system allow "specializations" of types? I.e. some way to define a special case for (Dictionary Integer b) that includes vectors?

_{Technically, dictionaries also include structures that implement gen:dict, but obviously generics don't exist in TR yet.}