This fixes an incorrect re-declaration of ed25519_sign.

Implement header files for some of the other library bits so that we don't need
to declare functions in crypto.cpp.

Replace hard coded references to the 32-byte key length with a constant, add utilities for copying data to and from fixed sized arrays

Rename axolotlpp as olm to avoid confusion with Axolotl-the-spec and Axolotl-the-OWS-libraries at moxie's request