Subdoc: Additional opcodes for Datastructure support

There has been some suggestions for additional subdoc opcodes, to simplify usage / make it easier to support Datatypes:

• Count how many elements exist in an array / dict
• Fetch of array slices.
• For subdoc mutations, create if doesn't exist (note that most subdoc commands require a path, so this may need some changes to existing semantics).
• ... maybe others ...