Fixes compilation errors in functions that are syntaxic sugar
to operate on serialized scalars.
Also make it explicit that square roots in fields whose size is
not congruent to 3 modulo 4 are not an error, they are just
not implemented yet.
Reported by @vitalonodo - Thanks!
For 25519, it's very likely that applications would ever need the
serialized representation. Expose the value as an integer as in
other curves. Rename the internal representation from `field_size`
to `field_order` for consistency.
Also fix a common typo in `scalar.sub()`.
After P-256, here comes P-384, also known as secp384r1.
Like P-256, it is required for TLS, and is the current NIST recommendation for key exchange and signatures, for better or for worse.
Like P-256, all the finite field arithmetic has been computed and verified to be correct by fiat-crypto.
