Refactor some internals of ff

[Pratyush]

Description

• Splits the contents of ff/src/fields/mod.rs into smaller files for easier management.
• Moves BitIterator out of ark_ff::fields and into ark_ff directly.
• Adds impl<'a, 'b> Add/Sub/Mul/Div<&'a F> for &'b F

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[weikengchen]

Looks good. See the documentation warnings (in GitHub/Files changed).

[weikengchen]

I believe this is the last "major" breaking change for arkworks-rs at this moment. I think we can merge it soon once the docs above are handled.