Follow-ups of `protocol` 0.15 integration

[igamigo]

#2095 had various comments after upgrading the protocol to version 0.15. #2132 addresses all of them, except for these remaining ones:

• conv.rs dedup: de-duplicate the conversion helpers shared between conv.rs and conv.rs, and rename the module to conversions.rs. (link)
• details_commitment design question: do we still need details_commitment, or can we drop the column? Investigate how we use it and potentially remove it. (link) <- ended up being addressed in fix: #2095 follow-ups #2132
• get_account() is expensive: get_account() deserializes the full Account on every partial delta; switch to applying updates instead of loading the whole account each time. (link)
• helpers.rs explanation: add comments explaining why these helpers live here instead of upstream. (link)
• Felt::new_unchecked safety: Felt::new_unchecked / Word::unchecked are used with Rng. Already tracked upstream in crypto#1025. link
  Related upstream tracking issue: Implement RandomCoin seeding from rand::rng crypto#1025

Some of these may require their own issue/discussion, but for now I'm writing them here to have the backlog up to date