Add the ed25519 curve

[weikengchen]

Description

Needs a decision from Pratyush on whether or not we want to add ed25519.

In addition, related to #115, if we add this curve, we probably should add the curve25519 curve soon, which is to avoid users from thinking ed25519 is curve25519's twisted Edwards curve.

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• Wrote unit tests
• Updated relevant documentation in the code
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[weikengchen]

@arkworks-rs/algebra This one is ready for review.

[weikengchen]

fixed the comments:

    /// Standard generators from https://neuromancer.sk/std/other/Ed25519.