Explore decoupling the generic `D` in plonky2's `ArithmeticExtensionGate` from the `D` in the `CircuitBuilder`

[ed255]

This would allow us to use extension gate arithmetic of a degree independent of the degree in the CircuitBuilder which is needed for the elliptic curve implementation.

For now we have our own constraints and gates for it.

This exploration would require sending a PR to plonky2.

See full context here #236 (comment)