Add signing class to the pk module

because emscripted has dropped support for the old flag and us
setting it is now breaking the build.

This is a workaround for a bug where signature verification would
overwrite the variable holding the signature.

This only happens on python2.

Signed-off-by: Damir Jelić <poljar@termina.org.uk>

OLMKit: Expose PK private key length

CMake support

OLMKit: Add objc wrappers for pk encryption/decryption

zero buffers in the JavaScript bindings

Poljar's improved python bindings

also reduce the amount of memory copying that we do