Ed25519: Improve tests

[Wollac]

• Use officially provided test vectors in https://ed25519.cr.yp.to/python/sign.input
• Add tests for all of the 14x14 = 196 critical inputs mentioned in https://hdevalence.ca/blog/2020-10-04-its-25519am
• Add tests that third party malleability is correctly rejected, i.e. reject values for S that are greater or equal than L (https://tools.ietf.org/html/rfc8032#section-5.1.7).

[nothingismagick]

can you confirm that this has been done to your satisfaction @Wollac ?

[Wollac]

I don't think anything has changed since I've opened this issues, as far as I know...

As far as I can see, ed25519.rs contains test vectors generated with utils/test_vectors/py/main.py but not those particular classes mentioned here. IMHO they would still be great to have.