Objective-C bindings

Currently, we don't provide compiled artifacts, therefore, manual steps to build and orchestrate the libraries and headers are necessary.

Running

Assuming that the header and library are in the same directory.

gcc ./main.m `gnustep-config --objc-flags` `gnustep-config --objc-libs` -L. -lfilecoin_signer_ffi -lgnustep-base -o ./main
LD_LIBRARY_PATH=. ./main