The python that searches the header files for exports isn't smart
enough to know what is a comment and what isn't, so it picks this
up too and emscripten then complains about it being undefined.
Change interface to allow the app to get the private part of the
key and instantiate a decryption object from just the private part
of the key.
Changes the function generating a key from random bytes to be
initialising a key with a private key (because it's exactly the
same thing). Exports & imports private key parts as ArrayBuffer at
JS level rather than base64 assuming we are moving that way in
general.