* 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.
Instead of having one struct with uninitialised fields
which are mutated, have explicit states and ensure that the types
show that there is no ambiguity which data is available or not.
Consequently, this removes quite a few `unwrap`/`expect` calls.
* Introduce NULL cipher and allow more configuration.
* Back to using the hash-code for handshake.
Using `Endpoint` would be incompatible with the existing protocol.
* Add comments.
