Add util support for Identity

(1) It would be good to have in org.opendaylight.yangtools.yang.model.api.type.IdentityrefTypeDefinition
to have a method that returns the IdentitySchemaNode, not just the name

(2) it would be good to have in IdentitySchemaNode a method that lets me get all the derived identities.
I need this since the identityRef references the base but I have no links down the identity tree.

for example in this Yang, I would be able to start at hashFunctionLeaf and
getIdentityNode() to get the IdentitySchemaNode.
From the base IdentitySchemaNode I can then recursively call getDerivedIdentities() to get BOB, IPSX and CRC and then from CRC to get CRC-Big and CRC_Small.

leaf hashFunctionLeaf {
type identityref

{ base "hashFunction"; }

identity hashFunction {
}
identity BOB

{ base "hashFunction"; }
identity IPSX { base "hashFunction"; }

identity CRC

{ base "hashFunction"; }

identity CRC_big

{ base "CRC"; }
identity CRC_small { base "CRC"; }

If this is already possible then please let me know how to do it