Add test vectors

[hdevalence]

The test vectors section of the website should be filled in.

The website has sections for both the ristretto255 and ristretto448 parameters.

For now I think it's fine just to add test vectors for ristretto255 and not ristretto448, since Decaf also works for Ed448, so there's not really a good reason to use Ristretto there.

Since mdbook allows rustdoc tests, we can make the test vectors executable using the curve25519-dalek reference implementation.

[hdevalence]

The website has test vectors, but they're incorrect (there was a bug in the script that generated them).

The test vectors should ensure that all the edge cases of the hash-to-group operation are exercised, which requires writing down how that operation works (#15, #25)

[hdevalence]

• Fix ristretto255 test vectors #31 fixes the incorrect test vectors
• there should be test vectors for the from_uniform_bytes that test the edge cases around stuff like high bits, how the reduction mod p works, etc.