Add basic BIER elements #561

Job	Run time
build (gcc, vanilla)	7m 15s
build (gcc, dpdk)	15m 38s
build (gcc, dpdk-light)	15m 43s
build (gcc, netmap)	7m 18s
build (gcc, vanilla-nopool)	7m 9s
build (gcc, dpdk-flow-dynamic)	15m 46s
build (clang, vanilla)	10m 45s
build (clang, dpdk)	17m 0s
build (clang, dpdk-light)	19m 0s
build (clang, netmap)	11m 21s
build (clang, vanilla-nopool)	10m 39s
build (clang, dpdk-flow-dynamic)	16m 40s
	2h 34m 14s

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