ffi: Implement ffivar.get()/set() methods.

Done by introducing another factored out helper API in binary.c. This API
can be reused also by array and struct modules.

examples/unix/ffi_example.py

@@ -19,6 +19,7 @@ print()
 perror("ffi before error")
 open("somethingnonexistent__", 0)
 print(errno)
+print(errno.get())
 perror("ffi after error")
 print()