Shorten the keyword polynomial-commitments to commitments

[weikengchen]

Description

A keyword is too long and is rejected by crates.io. Therefore, this PR shortens this keyword.

We could, however, change it into poly-commit, but it would overlap with the repo name. Therefore, commitments appears to be more meaningful.

---

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.
• Re-reviewed Files changed in the Github PR explorer

N/A:

• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md