Proto V2: improve the Context.Tree.kind API to not load values

This is a follow-up from @mbouaziz's comments about Context.Tree.kind on !2440 (merged)

This is also needed to help @smelc with the shallow trees API for manipulating Merkle proofs, were we want the values to also be lazily addressable.

As this is modifying sigs/v2/context.mli it would be great to get it reviewed and merged for Florence.