Implement EphemeralPrivateKey's for Orchard, Sapling, Sprout

[dconnolly]

Is your feature request related to a problem? Please describe.
To send money, we need an EphemeralPrivateKey to go with our EphemeralPublicKeys that already exist in zebra-chain.

Describe the solution you'd like
Implement the newtype wrappers around pallas::Scalar, jubjub::Scalar, and I think x25519-dalek::SecretKey for Orchard, Sapling, and Sprout resepectively.

Describe alternatives you've considered
Use those types directly, which elides the specific use and capabilities of these keys vs the other uses of those generic types other places in our codebase.

Additional context
This must be done to complete #269

[teor2345]

We're not planning to do full-node wallet work any time soon.

If we do, we'll need to decide if this ticket is in scope.