Reduce the need for extraneous generics in downstream code.

[Pratyush]

Description

Adds two main APIs:

• trait FieldWithVar: Field { type Var: FieldVar<Self, Self::BasePrimeField> }
• trait CurveWithVar<ConstraintF>: ProjectiveCurve { type Var: CurveVar<Self, ConstraintF> }

There are refactors to make this work. Primarily: we reimplement everything in terms of *_assign operators to prevent too many bounds everywhere.

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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.

• [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
• [x] 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

[weikengchen]

And remember to add a CHANGELOG entry.