This is a fork of libsignal-protocol-c, an implementation of Signal's
ratcheting forward secrecy protocol that works in synchronous and
asynchronous messaging. The fork adds support for OMEMO as defined in
XEP-0384 versions 0.3.0 and later.

OMEMO version 0.3.0 uses the original libsignal-protocol-c
implementation with its protocol versions 2 and 3.

OMEMO version 0.4.0+ is implemented using a new protocol version 4
internally.

In comparison with protocol version 3, it changes:
HKDF info strings
Protocol buffer encoding
Signature scheme (uses XEd25519 instead of custom
"Curve25519 signatures")
Specification-compliant double ratchet
Support for Ed25519 public keys
Various serializations
Removes unused functionality
