Prepared G2 consistency test

[weikengchen]

Description

This PR adds a consistency test for the g2 prepared element for BLS12-377.

There is space of improvement for this test: G2PreparedGadget at this moment only implements ToBytesGadget but not EqGadget, and therefore the code here converts them to bytes first and performs the checks, which is suboptimal.

Especially, this conversion seems to be extremely expensive, so that it cannot be used for MNT4/6-753. Therefore, the new test is not used on those curves.

This PR is related to arkworks-rs/r1cs-std#77

Since the bug for BLS12-377 will only be fixed once this repo is based on the newer release, it is expected that the test will fail until then, and this PR should only be merged in the next release.

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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.
• Wrote unit tests
• Updated relevant documentation in the code
• Re-reviewed Files changed in the Github PR explorer

N/A:

• Added a relevant changelog entry to the Pending section in CHANGELOG.md

[weikengchen]

Memory appears to be insufficient---now trying to use macos-latest instead.

[weikengchen]

Finally.

[Pratyush]

Thanks for this @weikengchen !!