bdiff: add version to help detect breaking binary changes Previously, we have no way to detect if a compiled .so file could be used or not, and blindly load it if it exists. Usually we carefully maintain compatibility of .so and fallback to pure code gracefully. But if we stick to the rules, certain nice changes will be impossible to make in a clean way. This patch adds a "version" constant to the module so we can detect inconsistency and take appropriate actions (warn, abort, fallback to pure, run make automatically) in module loader.