Allow `fn version()` to mutate version numbers in place.
Allow `fn version()` to mutate version numbers in place.

The `rust` allocated memory for the version no. needs to be modified by FFI.

In light of recent changes: we mark the variables as mutable, and make sure
the FFI accepts mutable raw pointers.

jedisct1/rust-zmq,pfernie/rust-zmq,petehayes102/rust-zmq,tempbottle/rust-zmq,dginev/rust-zmq,pblasucci/rust-zmq,erickt/rust-zmq,alexkamp/rust-zmq,cristicbz/rust-zmq