Add a Field::from_base_field_elements method

[ValarDragon]

Description

This PR adds a Field::from_base_field_elements() method. This is useful for writing hashing to curve code that is general over the base field of the curve.

Furthermore, this PR also fixes a type safety issue with field extensions, where an extension fields basefield may not extend from the base prime field. (Needed for this PR to compile)

To test this, I've added an fq2 and fq6 to test_fields.

closes: #163

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• 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
• 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]

This LGTM. Quick question: since you’re editing the cubic extension file, I noticed there are some incorrect comments talking about quadratic extension. (Eg around lines 29 and 31) could you fix these also? Thanks :)

[ValarDragon]

Fixed