Licensing about elligator

[LoupVaillant]

• The reference source code is all "public domain" (CC0/2_BSD), so we should be good there.
• I believe the test vectors are not copyrightable. even if they were, they come from public domain code.
• Code snippets in the explicit formulas page are intended for unrestricted use. I also suspect that given their nature (short examples in teaching material), they may not be copyrightable. Still, it might be worth adding a note at the top of the page stating that they're licenced under CC0 and 2_BSD (with a link to a dedicated licence file).
• For the rest of the documentation, I'd rather retain our copyright for now (thus, "no licence"). I'm okay with the idea that we may need to change it later, even if it means leaving this issue open indefinitely.