Improve organization of various helpers once miden-crypto is merged into the repo

This crate doesn't depend on miden-core, only miden-serde-utils, and we don't want it to depend on miden-core (I suspect it can't in any case, due to cyclic dependencies). I'm not sure why these helpers were added to miden-core rather than miden-serde-utils (probably because it would've been too much of a pain to do the release cycle dance), but these can be removed in favor of ones in miden-serde-utils once we can move them from miden-core to miden-serde-utils and have miden-core just re-export them.

For now this was the only way to get those helpers where they were needed.

Originally posted by @bitwalker in https://github.com/0xMiden/miden-vm/pull/3398#discussion_r3626097131

Now that the miden-crypto repo is being integrated into the miden-vm repo, we can migrate some helpers around to avoid some unnecessary duplication that was introduced just to avoid the dependency management pain:

• miden-core defines some useful serialization helpers under its serde namespace that should be moved to miden-serde-utils
• This will allow duplicate copies of those helpers in miden-utils-indexing to be removed in favor of the ones in miden-serde-utils (as it does not/cannot depend on miden-core)

There may be other similar duplication that we should document/tackle as part of this issue. I could not land the two above on main because the merge hasn't happened there yet, so these changes will need to be implemented against next