Change the default field type of the pairing var

[weikengchen]

Description

When fixing the groth16 library, it seems that the default field type of PairingVar is insufficient---we need to use the BasePrimeField. This PR makes an attempt, and will be finalized when we know if it fixes the issue in Groth16.

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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.
• [x] Re-reviewed Files changed in the Github PR explorer

N/A:

• [ ] Wrote unit tests
• [ ] Updated relevant documentation in the code
• [ ] Added a relevant changelog entry to the Pending section in CHANGELOG.md

[weikengchen]

I will wait a bit to merge---I am fixing the code in Groth16 and I am not fully sure if this is needed.

[Pratyush]

I pushed some fixes there too