Improve InstanceIdentifier definition

Current InstanceIdentifier definition (the yang-binding one) has major deficiencies in these areas:

• it exposes a List<> of path elements, which means that deriving identifiers requires copy operations
• one can instantiate essentially untyped identifiers
• there is no provision for quick "child of this identifier" things
• there is no difference between a key-aware and keyless target
• there is no difference between a wildcard and non-wildcard identifier