Method arity almost completed

This PR implement almost all cases for `Method#arity` except `respond_to_missing?` because it requires implementation of `Kernel#respond_to_missig?`.

But I have some concerns about adding
```
_immutable_fields_ = ["name", "w_class", "block", "bytecode"]
```
I did that to fix translation. Without `block` and `bytecode` it wouldn't compile. Maybe you know better solution?