Add instance-identifier and leaf-reference native types

We need a bridge between yang-model-api and yang-data-api where types are concerned. Specifically, we need to map TypeDefinitiion.getDefaultValue() to native types, as expected by yang-data-api.

We can generalize YangInstanceIdentifier, but its deficiencies must be fixed first:

• augmentation cross-over happens on namespace change, without an explicit node, e.g. AugmentationIdentifier is actually QNameNamespace different from parent
• ability to target via list order, for user ordered lists and leaf-lists
• it really needs to align to the XPath view of yang-modeled data

More impact/tradeoff analysis is required.