Support simple closures as Obj-C method arguments

Only pointers to functions which don't take any paramters and return void are
supported for now. To invoke Function0.invoke() method we use the Closure API
from libffi, binding a generic closure handler with specific arguments (like
JavaVM, function object, etc.). To build KotlinNative now you need a ffi/
directory containing libffi.dylib and ffi.h, this can be created with ffi_make
script for now, it builds libffi from source. We need to do this, since the
standard version of libffi on Mac OS X is way too old and the Closure API has
changed significantly since then.
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