Fix instance-identifier mapping

We currently map YANG's instance-identifier to yang.binding.InstanceIdentifier. Unfortunately InstanceIdentifier is not able to represent all valid instance-identifiers, as it cannot address leaf nodes.

Design a replacement for InstanceIdentifier, BindingInstanceIdentifier, which will have two basic specializations:

• BindingDataObjectIdentifier, which is essentially InstanceIdentifier, but does not allow wildcards
• BindingValueIdentifier, which extends InstanceIdentifier with the last leaf step, in the form of a accessor

This latter mirrors how we identify leaf values in Binding Query, so there is a precedent.