Early block returns are required in order to break out of tail recursive
methods. For example, Block.while_true would be implemented as follows:

    impl Block {
      def while_true(block: ()) -> Nil {
        call.if_false { return }
        block.call
        while_true(block)
      }
    }

Without the ability to perform an early return this method would never
return. Block returns act much like Smalltalk's block returns: they
return to the scope that defined the block. Internally this works by
taking the Binding of the current scope, finding it's top-most parent,
then unwinding until we reach the execution context that uses this
binding.

I opted for extending the VM's "Return" instruction instead of adding a
separate one as otherwise a lot of logic would have to be duplicated.