Refactor `ToIpld` to `ToLurk`

[winston-h-zhang]

Relevant Zulip link

TL;DR We are replacing the ToIpld serialization pipeline with ToLurk instead. Because Lurk supports functional commits, its code is naturally content-addressable data as well. Hence we can use Lurk as our content-addressing backend instead of IPLD. In particular, we must

• Replace ToIpld with ToLurk everywhere
• Replace replace cid with Lurk field elements and IpldToCid with our own Poseidon hash implementation (mirroring lurk-rs's commit).
• Rewrite our current .yatima_store pipeline to mimic lurk-rs's ScalarStore (and probably call it .lurk_store)

[arthurpaulino]

We don't have a .yatima_store yet