* Consolidate keypairs in core.
Introduce the concept of a node's identity keypair in libp2p-core,
instead of only the public key:
* New module: libp2p_core::identity with submodules for the currently
supported key types. An identity::Keypair and identity::PublicKey
support the creation and verification of signatures. The public key
supports encoding/decoding according to the libp2p specs.
* The secio protocol is simplified as a result of moving code to libp2p-core.
* The noise protocol is slightly simplified by consolidating ed25519
keypairs in libp2p-core and using x25519-dalek for DH. Furthermore,
Ed25519 to X25519 keypair conversion is now complete and tested.
Generalise over the DH keys in the noise protocol.
Generalise over the DH keys and thus DH parameter in handshake patterns
of the Noise protocol, such that it is easy to support other DH schemes
in the future, e.g. X448.
* Address new review comments.
