Includes a MessageNano subclass's name's hashCode in hashCode calculations.

In the current implementation, a message with the same amount of null or
equal-valued fields as a different message type will have the same
hashCode. This adds more variety by including the hashCode of the
class's name in the hashCode calculations.

Change-Id: I284e3e6d198ad8037815948d1f65686465ffd623
Signed-off-by: Jason Neufeld <jneufeld@google.com>