Namespaces for arrow functions are generated non-exclusively

[alfredsgenkins]

Describe the bug

The namespace generated for arrow functions of then handler do not generate namespaces correctly.

[yeegor]

I can propose several solutions to this issue:

1. Less backwards-compatible, more straightforward

2. More backwards-compatible, less straightforward

Since we don't have many plugins for v4 now, we can allow breaking changes to some extent. What do you think?

[alfredsgenkins]

I prefer first option, while the question is what is that all keyword appearing in the code? Are there other potentially breaking cases, except then? Probably, there are. In that case, maybe we should add one, two, etc. (or first, second, ...) (or 1, 2, ...) to arguments? Otherwise we must handle each case separately.