[FEATURE] Extending operator declarations with type annotations

[konnov]

This topic pops up from time to time. Sometimes, we have to annotate operators, e.g., with types or the sets of assigned variables, #226 and #261.

It looks to me that annotating expressions is a too fine-grained approach. On the other hand, we can annotate operator definitions. I see two options:

1. Add annotations directly in TlaOperDecl,
2. Similar to TlaEx, introduce a uid in TlaOperDecl and store annotations as maps from uid to whatever.

I think the second approach is more flexible and extendable, but we will have to extend TransformationListener.

@Kukovec what is your opinion?

[konnov]

Duplicate of #262