feature: rate limit

[SkohTV]

Fix #215

Hi, sorry for the long hiatus,
A bunch of stuff came up, and I didn't have the time to do the big rebase+refactor.

Changes:

• Did the big rebase.
• Changed ratelimit from an action to a matcher, supports for eq with an uint32
• Kept the map, because some persistence is required for rate limiting.
• Also kept and slightly changed the elfstub.

	10 first	All after
chain my_chain BF_HOOK_XDP{ifindex=2} DROP rule meta.ratelimit eq 10 ACCEPT	ACCEPT	DROP
chain my_chain BF_HOOK_XDP{ifindex=2} ACCEPT rule meta.ratelimit eq 10 DROP	DROP	ACCEPT
chain my_chain BF_HOOK_XDP{ifindex=2} DROP rule meta.ratelimit not eq 10 ACCEPT	DROP	ACCEPT
chain my_chain BF_HOOK_XDP{ifindex=2} ACCEPT rule meta.ratelimit not eq 10 DROP	ACCEPT	DROP

I'd really like to get this PR over the finish line at some point :)

[qdeslandes]

I went for an early review, mostly for the parsing part. On the BPF side, the map will contain the runtime values for a given rule's rate limit (current burst/allowance, limit, last update time...).