Make the to_bytes/from_bytes round trip of ExpandedSecretKey idempotent

[poljar]

A mistake snuck in with 9b166b7. The new scalar_bytes field of the ExpandedSecretKey is not used in the ExpandedSecretKey::to_bytes() method, instead the, now clamped, scalar is used.

This in turn means that the following snippet won't produce the same ExpandedSecretKey:

let key = ExpandedSecretKey::from_bytes(bytes);
let another_key = ExpandedSecretKey::from_bytes(&key.to_bytes())

Down the line, it means that serializing/deserializing this struct will produce differing keys and thus spurious signature verification errors.

[rozbb]

Woah great find. I think this merits a new release. @tarcieri ?

[poljar]

Ah, there was a slight copy/paste issue with one of the error messages. Amended with a fixup commit.

[tarcieri]

I think this merits a new release. @tarcieri ?

@rozbb sure, why not

[rozbb]

@poljar Sorry for the bother, but we just moved ed25519-dalek over to the big monorepo. Would you mind rebasing the PR for there?

[poljar]

@poljar Sorry for the bother, but we just moved ed25519-dalek over to the big monorepo. Would you mind rebasing the PR for there?

Sure, no problem.

edit: Done -> dalek-cryptography/curve25519-dalek#541

[poljar]

Closing as this has been transferred.