Introduce higher level API to SMI objects

[etingof]

This problem came up lately, perhaps we could have some higher-level API to achieve this and similar functionality around indices easier:

for var_bind in var_binds:
    object_identity, value = var_bind
    mib_node = object_identity.getMibNode()
    object_instance_oid = object_identity.getOid()
    object_oid = mib_node.getName()
    index_part = object_instance_oid[len(object_oid):]