BUG-1440: extract a reusable StringInstanceIdentifierCodec

Eliminates the helpers package and exposes the mapping utilities in the
yang-data-util package. The utilities can then be reused by the XML
implementation, too.

Change-Id: I20df1b04e6ad875d13d18072852b0f5ac35ab8cc
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1440: extract a reusable StringInstanceIdentifierCodec"