Add Pluto-Eris cycle of curves

[davidnevadoc]

Implementation of the half-pairing cycle of prime-order curves described here: https://github.com/daira/pluto-eris

Pluto is a Barreto–Naehrig curve and its pairing implementation closely resembles that of bn256 curve. See the Optimal Ate pairing described in the article: https://eprint.iacr.org/2010/354.pdf.

The constants for the fields and its extensions are derived these notebooks: https://github.com/davidnevadoc/ec-constants/tree/main/pluto_eris

[CPerezz]

First of all, nice work. It's really cool to get these curves implemented!

I have a couple of things that have written down while reviewing:

• README.md needs to be updated mentioning that pluto/eris curves are supported.
• All tests should be inside test modules please.
• Add comments for functions left unimplemented. Why were they unimplemented?
• Add a macro for all Fq/p/p12/6...Repr structs. All of them impl the same things over and over. So it's just boilerplate that we can save up.
• All of the things that are publicly exported but not docummented should do the exercise of: 1. Think if they need to be exported. 2. if so, add docs. Otherwise change visibility.

[davidnevadoc]

I have addressed the comments relevant to this PR. However there are still some unresolved that I think should be fixed in a different PR. Mainly:

1. Tests and benchmarks
   Fixing these consistently would involved modifying other curves which is outside of the scope of this PR. ( Just added the test vectors specific for this curves). This should be fixed in Upgrade testing across the lib #76
2. Documentation and unimplemented functions
   There are some unimplemented and undocumented functions. These are unimplemented or undocumented in bn256.
   I plan to fix both issues in a single PR to avoid duplicating work. I also would like to introduce a common macro for the BN pairing and do some refactoring to improve the code quality in Refactor BN curves. #105

[CPerezz]

We are inconsistent between our comments.

We have the hex representation of only some values for the constants that are defined in this PR.

I think we should either have them all or none. But it's quite confusing to only have "some of them".

We can address in a follow-up PR if you want. But this has to be addressed at some point.

Other than that, LGTM!

Good work with this!!

[CPerezz]

cc: @daira this might be of your interest!