Fix: Object typing for bidirectional effects

[dvdvgt]

This issue is to close #152.

Description

Check #152 for details.

Tasks

• Get simple example to compile
• Add block parameters to operations
• Write tests and find issues with current implementation
• Discuss solution for Check return type annotation of object operations #157

Changelog

• Breaking: Deprecate effect ... { ... } notation in favour of interface ... { ... }. effect ... is now only allowed for singleton interfaces.
• Add support for block parameters for operations

[b-studios]

This was probably added by accident in IntelliJ

[b-studios]

I minimized one of the failing tests to

interface Compose {
    def compose[A, B] {g: A => B}: A => B at {g}
}

def handler {prog: () => Unit / Compose} =
  try { prog() } with Compose {
    def compose[F, G]() = ()
  }

def main() = handler { do compose[Int,  Int] { x => 1 }; () }

In fact, bidirectional handlers are not yet implemented in backends other than JS (I don't know about LLVM, TBH; @phischu ?).

[b-studios]

I think it is fine to ignore the failing test for the ML backend for now and document it as "mutual recursion not supported".