Simplify cache logic

Tell the cache logic where we want to track things in the cache. Then have the cache logic decide on the rules of wether or not we should actually track it based on string length. This leads to a simpler code path for cache logic.
Remove the "Ctx" constructor from the Cache.t type

The module is fairly simple and it is quite easy to figure out what the constructor is and how it
operates. Further, since the types in the pair are different, there is no risk of messing them up
and binding them to each other in a wrong way.