Recursion check in operator definitions

[bugarela]

As recursion is not allowed, we should detect recursion in operator and lambda definitions to report them as unsupported. This should be a simple occurs check of the operator's name in its body.

[bugarela]

We should probably do it before type checking.

Maybe correlates with #129 and #177

[konnov]

I just ran into this issue by typing a wrong name and introducing infinite recursion. The plugin feedback was too complex for me to understand. It would be great to have this fixed :)

[bugarela]

Minimal example:

module foo {
  val a = a
}

I'll wait until the refactor of the lookup system to fix this because this depends too much on it and I will have to completely re-do the check when it changes.

[konnov]

Handled by #1008, including the MWE