Support for the Pluto-Eris half-pairing cycle

[daira]

Description

Add support for the Pluto and Eris curves. These curves form a half-pairing cycle described in https://github.com/daira/pluto-eris . Currently, the pairing for Pluto is not implemented.

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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)
• [ ] ~~Linked to Github issue with discussion and accepted design~~ OR have an explanation in the PR that describes this work.
• [x] Wrote unit tests
• [x] Updated relevant documentation in the code
• [x] Added a relevant changelog entry to the Pending section in CHANGELOG.md
• [x] Re-reviewed Files changed in the Github PR explorer

[daira]

Note that this depends on https://github.com/arkworks-rs/algebra/pull/261 (which has now been merged, but is not in a release).

[daira]

https://github.com/arkworks-rs/algebra/issues/274 will be needed in order to support Pluto's pairing.

[weikengchen]

Updated that arkworks-rs/algebra#274 would no longer be a blocker.