Work with PkDecryption keys by their private keys

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WebAssembly support

Mostly because the standard emscripten docker image does not have
libjson-perl, but python always comes with json. But also because
it was impenetrable.

Call appropriate wrapper function

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.

Remove trailing letter 'K's from the test pubkeys

base64 encoded newlines somehow?

Don't think this matters since there's no PkEncryption /
PkDecryption object being passed, but for the sake of consistency