Requires IsKeyedObjectContainer in TupleTree.h wherever possible,
instead of IsContainer. This enables using std::vectors.
In this way, they will not be diffed based on the key, but on the entire
vector. This is intended.
This commit drops the KeyTraits in favor of a std::any-like solution.
Basically, we type erase any key the user wants to employ, just exposing
a virtual version of the destructor, a comparison operator and a clone
primitive.