[Do not merge, RFC]Add dataflow!

With futures, you can call ```value!``` if you want exceptions to bubble up. It'd be rather nice if you could do the same thing with dataflow.

This is the simplest possible solution, if you collectively think this is a worthwhile feature I'll polish it up, add tests, etc.

Author: mastfissh

lib/concurrent/dataflow.rb

@@ -88,4 +88,33 @@ def dataflow_with(executor, *inputs, &block)
     result
   end
   module_function :dataflow_with
+  
+  def dataflow!(*inputs, &block)
+    dataflow_with(Concurrent.configuration.global_task_pool, *inputs, &block)
+  end
+  module_function :dataflow!
+
+  def dataflow_with!(executor, *inputs, &block)
+    raise ArgumentError.new('an executor must be provided') if executor.nil?
+    raise ArgumentError.new('no block given') unless block_given?
+    raise ArgumentError.new('not all dependencies are IVars') unless inputs.all? { |input| input.is_a? IVar }
+
+    result = Future.new(executor: executor) do
+      values = inputs.map { |input| input.value! }
+      block.call(*values)
+    end
+
+    if inputs.empty?
+      result.execute
+    else
+      counter = DependencyCounter.new(inputs.size) { result.execute }
+
+      inputs.each do |input|
+        input.add_observer counter
+      end
+    end
+
+    result
+  end
+  module_function :dataflow_with!
 end