* ed25519-dalek: remove `ExpandedSecretKey::to_bytes`
The reason `ExpandedSecretKey` needs a private `scalar_bytes` field is
to retain the canonical scalar bytes as output by SHA-512 during key
expansion so they can be serialized by the `to_bytes` method.
However, `ExpandedSecretKey`s should not be serialized to the wire.
Removing this method allows the private field to be removed, which
allows `ExpandedSecretKey` to be constructed entirely from public
fields. This provides an alternative to #544 for use cases like
Ed25519-BIP32 where the private scalar is derived rather than clamped
from bytes.
One other change is needed: `to_scalar_bytes` was changed to `to_scalar`
as the canonical scalar bytes are no longer retained, however this has
no impact on its main use case, X25519 Diffie-Hellman exchanges, where
the `Scalar` should NOT be written to the wire anyway.
* Added scalar byte comparison back to ed25519-dalek x25519 test
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Co-authored-by: Michael Rosenberg <michael@mrosenberg.pub>
