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

since they're not for the right repository any more

- build both Python2 and Python3 libs by default, and add separate rules
  building Python2 and Python.
- use the libraries as built by setuptools, rather than building again
  separately

The library can now be installed using CMake v3.0+.

Below is an example configuration.

1. Generate configuation

cmake -H. -Bbuild
    -GNinja
    -DCMAKE_BUILD_TYPE=Release // The default profile.
    -DCMAKE_INSTALL_PREFIX=/usr/local/
    -DBUILD_SHARED_LIBS=ON
    -DOLM_TESTS=1
    -DOLM_FUZZERS=1

2. Build & install the targets

cmake --build build --config Release --target install

3. Run the tests

cd build/test && ctest .

The library can also be used as a dependency with CMake using

find_package(Olm::Olm REQUIRED)
target_link_libraries(my_exe Olm::Olm)

Signed-off-by: Konstantinos Sideris <sideris.konstantin@gmail.com>