Remove ATE_LOOP_COUNT_IS_NEGATIVE from BN curve parameter trait

[weikengchen]

Description

In #275, Daira pointed out that the current BN parameter trait, which requires X_IS_NEGATIVE and ATE_LOOP_COUNT_IS_NEGATIVE, can be improved.

Because for BN curves, the ate loop count is 6x+2, so ATE_LOOP_COUNT_IS_NEGATIVE == X_IS_NEGATIVE always hold.

Therefore, we may want to keep only one of them. It seems easier to remove ATE_LOOP_COUNT_IS_NEGATIVE. This PR makes this change.

To avoid breaking, this PR does not delete ATE_LOOP_COUNT_IS_NEGATIVE but adds a default implementation:

const ATE_LOOP_COUNT_IS_NEGATIVE: bool = Self::X_IS_NEGATIVE;

This is to avoid breaking ark-curves. Any thoughts? Should we remove it directly and add a pending PR for ark-curves? I will wait for people's opinions on this and then decide what changelog entry to add ("improvement" or "breaking").

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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 (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• 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

N/A:

• Wrote unit tests

[Pratyush]

Let's remove it; I doubt anybody except ark-curves (and maybe ark-r1cs-std) is relying on this.

[weikengchen]

This PR is better merged right before the next release because merging this PR would cause all tests against curves to be "canceled" and may affect development.

Added the merge-in-next-release label.

[daira]

Untested ACK

[ValarDragon]

LGTM

[weikengchen]

(Convert it into the draft to avoid accidental merging)