Prepare input datagram handling code for pure C implementation of
input datagram writing.

	modified:   src/bruce/input_dg/any_partition/v0/v0_input_dg_constants.h
	modified:   src/bruce/input_dg/any_partition/v0/v0_input_dg_reader.cc
	modified:   src/bruce/input_dg/any_partition/v0/v0_input_dg_writer.cc
	modified:   src/bruce/input_dg/input_dg_common.h
	new file:   src/bruce/input_dg/input_dg_constants.h
	modified:   src/bruce/input_dg/input_dg_util.cc
	modified:   src/bruce/input_dg/old_v0_input_dg_writer.cc
	modified:   src/bruce/input_dg/partition_key/v0/v0_input_dg_constants.h
	modified:   src/bruce/input_dg/partition_key/v0/v0_input_dg_reader.cc
	modified:   src/bruce/input_dg/partition_key/v0/v0_input_dg_writer.cc