Add derived impl of squaring for MontBackend

[Pratyush]

Description

Implements unrolled squaring algorithm for prime fields via the MontConfig macro. Incorporates the fix from #526.

---

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

[mmagician]

1. Where does the unroll actually happen? Is this only due to the attribute macro on the trait definition? If that's the case then already the default impl should be getting unrolled no?
2. What's the advantage of having two implementations, where the default trait impl and the one derived from macro are the same, as far as I can see? I think I might have missed some optimization step.