Add a Montgomery representation of Curve25519, called "Curve25519"

[pierluca]

As part of #384 and #528, the group/-directory has been cleaned up for the v4 release.

Paraphrasing @ineiti in #384 :

#384 was triggered by a question that came up with ByzGen about our use of Curve25519. This curve can be represented in two different ways:

• Twisted Edward Curve: https://tools.ietf.org/html/rfc8032
• Montgomery Curve: https://tools.ietf.org/html/rfc7748

The two representations are isogenic, which means that you can get from one representation to the other with a change of coordinates.

The two (three) curve25519 implementation in kyber are as follows:

• Ed25519 in group/ed25519 is either constant-time, or variable-time, Twisted Edwards Curve implementation
• Variable Ed25519 in group/var_ed25519 is a variable-time, Twisted Edwards Curve implementation

As of 2019, it seems that a consensus is materializing, where Edwards25519 refers to the Twisted Edwards representation, while Curve25519 refers to the Montgomery representation.

➡️ We should add a Montgomery representation of Curve25519 to the repo and call it Curve25519