Improve curves high-level structs and traits

[bgrieder]

The implementing structs and traits for curves need to be revisited so that we can have high-level traits that cover all the possible curves.
There should be 4 of them.

• One for a Scalar
• One for a Private Key (which can be built from a Scalar)
• One for a (Curve) Point (which can be built from a Scalar)
• One for a PublicKey (which can be built from a Curve Point and a PrivateKey)

Also, let us try to use associated types and const as much as possible here since:

• these traits are linked
• const generics "leak" everywhere

[bgrieder]

To support RSA:

• PrivateKey should only be built from a CS random and a PKCS8 (a Zeroizing Vec ?)
• PublickKey should be built from a PrivateKey only

The types described above should be renamed EcPrivateKey and EcPublicKey

The most commonly accepted way of creating a Public key seems to be calling public_key() on the Private Key.
This will trigger a redesign of NistPrivate/PublicKey