Fixes https://github.com/google/nftables/issues/154
Added support for intervals in concat sets
Added missing constants, Concatenation flag and KeyEnd field to Set type with marshaling support
Added ConcatSetTypeElements function to derive base types from concatenated types
Changed nftDatatypes list to map
Added tests
Based on https://git.netfilter.org/nftables/commit/include/datatype.h?id=cca4c856301caa8959ac98aac5811130bc19512c
It makes more sense to point to datatype.h in the comment since the
actual definitions of these types are scattered throughout src/.
uid_t and gid_t sizes are fixed at 4 bytes currently, but that might
of course change. I couldn't find a good way to make this
dynamic. There are a number of Sizeof* constants in x/sys/unix, so
that's probably the right place to add them.