Add element(i: usize) to Domain trait

[ValarDragon]

Description

Adds a function get_element(i) to the Domain trait, which returns the ith element of a domain. Such a method is useful in many applications, including STARKs.

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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 (main)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests - Did not, I view this as sufficiently simple at the moment
• 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