[TS] Handle constructors in the list of methods #319

We need to carefully handle the presence of the constructor in the list of methods (EtsClass::methods).

• If we receive the IR (from AA) where methods contain a non-empty constructor, we are good.
• If there is no explicit constructor in the TS class, AA will create --- in the most cases --- an empty one (on their side, it is also marked as "synthetic"), and we will receive it via IR.
• However, sometimes there is no constructor among methods in the IR. We handle this inside EtsClass::ctor by searching for the method called constructor among the methods, and if there is no such method, we return (in val ctor) a newly (lazily cached) created one. Probably we should consider adding this newly created ctor in methods list. Or better just ensure that methods list contains the constructor (or put a synthetic one) during the initialization of EtsClassImpl.