Expose a way to check if a value has been seen before

[w4]

We store hashes of each direct descendant's Storkable::value in each Storkable as a fix for #1 and we don't expose this to consumers. Right now consumers have to create a flat Vec and hash each Storkable::value themselves to check if they've seen the link before - even if they've all come from the same parent.

We need to figure out a good API to see if a "value" has been seen before from any parent, for example if the heirarchy looks like this:

root
↳ 123
 ↳ 456
  ↳ 789
↳ abc
 ↳ def
  ↳ ghi

We'd expect root.seen(789) and 123.seen(456) to be true but 456.seen(123) or abc.seen(123) not to be.

Right now children only hold references to their parents and not vice-versa so this lookup can currently only be done in reverse.