Don't require inversions for reverse lambdas

[TomasMikula]

The reverse lambda, i.e. producing in

 λ { (a: $[A]) =>
  producing { (b: ??[B]) =>
    // ...
  }
} : A -⚬ B

where b is a placeholder for the out-port, is currently implemented via a forward lambda on an inverted expression (??[B] = $[-[B]]).

This has 2 drawbacks:

• It is only available if the the underlying DSL (-⚬) has inversions.
• The resulting point-free AST contains extra inversions.

However, reverse lambdas can be implemented without inversions. Note that it will require some changes (generalizations) in the implementation of lambda translation, though.