Memory retain cycles

[PeterKeDer]

Find and fix memory retain cycles where two (or more) Rc are pointing in a cycle, making them never released. This can be fixed by changing some Rc into Weak such that strong reference with Rc is only "one-way".

Examples:

• A function owns the environment in which it is declared, and that environment also owns the function.

  For example

  {
      // Environment "A" owns foo
      fun foo() {
          // foo's closure references environment "A"
      }
  }
  // Both "A" and foo are not dropped

  Consider changing DoveFunction.closure from Rc to Weak.