Introduce HashCodeBuilder

Introduce HashCodeBuilder

This patch moves HashCodeBuilder, and internal yang-binding utility
class, into the util package and make it public.

The class was originally used to incrementally create InstanceIdentifier
hash code. As it turns out, we have at least two other potential users,
who could benefit from that same pattern: SchemaPath and the data-api
InstanceIdentifier, since they really are just different views on the
same concept.

Change-Id: I6a3208de213a47a2ed98e64f73a883dca73034fc
Signed-off-by: Robert Varga <rovarga@cisco.com>