Implement optimized vector of CIDs

[lemmih]

Issue summary

We have optimized versions of HashSet<Cid> and HashMap<Cid, _>. They use the Struct of Arrays technique to remove fields the CIDs have in common. We should do the same for vectors of cids.

• Implement CidVec.
• Use CidVec in TipsetKeys.
• Ask in channel about what to do with TipsetKeys::cids.
• Write quickcheck properties to make sure CidVec is identical to Vec<Cid>.

Other information and links

[jdjaustin]

Closed by #3275