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Makes `PrivateKey` take actual private keys (#737)

* Makes `PrivateKey` take actual private keys

Before, it would store their seed bytes in a `Secret`.
While zeroization helps with some attacks, we didn't leave it to the
cryptography implementation to further protect the information.

This introduces a new `Env` type dependency in `PrivateKey`.
Instead of forwarding it to both types that store `PrivateKey`, namely
`CredentialSource` and `PublicKeyCredentialSource`, we make them hold a
wrapped version of that private key. This has several advantages:

- We now iterate over stored credentials and filter them more easily
  without having to decrypt the private key material.
- Private keys wrapping moved out of high level data types, and closer
  to the cryptographic implementation.

The latter is useful for removing `to_slice` in `SecretKey`, and
replacing it with a more generic `wrap`, so that private key material
does not have to leave hardware cryptography.

The second goal this PR prepares for is to move Ed25519 into `Env`'s
crypto API.

* Slightly more precise `PrivateKey` API

* Removes unnecessary explicit deref

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