Add basic BIER elements #606

Job	Run time
build (gcc, vanilla)	8m 5s
build (gcc, dpdk)	8m 48s
build (gcc, dpdk-light)	11m 5s
build (gcc, netmap)	7m 52s
build (gcc, vanilla-nopool)	7m 47s
build (gcc, dpdk-flow-dynamic)	9m 0s
build (clang, vanilla)	10m 44s
build (clang, dpdk)	11m 46s
build (clang, dpdk-light)	13m 38s
build (clang, netmap)	10m 36s
build (clang, vanilla-nopool)	10m 52s
build (clang, dpdk-flow-dynamic)	11m 46s
mirror	41s
	2h 2m 40s

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