Give a load of internal symbols "_olm_" prefixes. This better delineates the
public and private interfaces in the module, and helps avoid internal symbols
leaking out and possibly being abused.

These seem to be the wrong way around.

We don't want anything which does dynamic memory allocation in the library.

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