Generate unions generate getValue()

Unions have a synthetic getValue() method, which returns a char[], as well as a corresponding constructor. This was needed for Config Subsystem and its use of JMX and is superfluous with that component gone.

Furthermore the synthetic method encroaches on method namespace which is populated by names from model - hence some constructs would end up not being generated correctly.

Fix this by generating a stringValue() synthetic method, without the ceremony in binding model itself. Union objects can still be instantiated using their associated Builder's getDefaultInstance() method.