Implement sharing for hls-graph Keys

[wz1000]

On large projects, duplicate Keys start to comprise a significant (~5-10%) fraction of the heap. This becomes better if we use Ints as keys and de-duplicate the actual values by storing them in maps. Keys are mostly just compared and hash during usual operation, printing them out is rare so a lookup shouldn't pessimize it much.

Also use HashSets to keep track of dependencies. However, this means that the underlying value for Keys will never be garbage collected once allocated.