Fix for add_many (issue #23)

[tgodden]

Description

Fixed issue #23.

Explanation

An addition of two n-bit number can at most be an n+1-bit number. So for m n-bit numbers, the result will be at most ceil(log2(m)) + n bits long.

I am not sure how to write these unit tests, because group size varies drastically between tested curves, any suggestions are appreciated.

• [x] Targeted PR against correct branch (master)
• [x] 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

[Pratyush]

Thanks for the fix!

[Pratyush]

Re: tests, we don't need to test every edge case (as that is infeasible in this case anyway), so we can just focus on a good number of inputs (say, 25), and a single field (say bls12_381::Fr).