Change: the implementation of coroutine and move it to modules/coroutine;
Change: asynchronous class to asynchronous object;
Change: class decorator and the prefix/suffix rules;
Change: @@{} and @@(){} to frame{} and frame(){};
Add: (bring back) assertion operators "e1 ?? e2";
Fix: a type checking bug for nested code section methods;
Remove: alias keywords "sub", "function" and "continue".