ed25519-donna refactoring

[onvej-sl]

It's pain to write code using ed25519-donna since:

• It's a one-purpose library aiming only at signing, verifying and performing Diffie-Hellman key exchange.
• It has almost no abstraction for other functions (especially for private and public key operations).
• It uses several number and point representations.
• It's badly documented. In particular, it's not clear what representations of inputs and outputs of functions are used.
• It's difficult to review any code using the library.

I suggest creating a well-documented abstraction for private and public key operations.

[ZdenekSL]

Out of scope for now.