`SmallCid` internals for collection types

[aatifsyed]

CidHashMap was changed from this:

struct CidHashMap<V> {
    small: HashMap<[u8;32], V>,
    big: HashMap<Cid, V>
}

to this:

struct CidHashMap<V>(HashMap<SmallCid, V>)

in #3388

That's wrong - we still care about saving overhead per-cid.

we need to restructure those changes, which looks like this I think:

• SmallCid stays, for use in FrozenCid. It's the right tradeoff here
• Have a public CompactCid, which is the implementation detail for SmallCid , for use in collection types

[aatifsyed]

We’re already bound by memory when manipulating Forest.CAR files. The size of the files we can handle is entirely determined by the available memory. Using 25% more memory means we can only generate Forest.CAR files that are 25% smaller.

• @lemmih, in Slack

So we care about this

[aatifsyed]

Using two HashMaps

Using HashMap<SmallCid, V>