Helper contract to support initializer functions. To use it, replace the constructor with a function that has the initializer modifier. WARNING: Unlike constructors, initializer functions must be manually invoked. This applies both to deploying an Initializable contract, as well as extending an Initializable contract via inheritance. WARNING: When used with inheritance, manual care must be taken to not invoke a parent initializer twice, or ensure that all initializers are idempotent, because this is not dealt with automatically as with constructors.