[core] Refactoring IOFormats so there is one and only obvious way to send it.

This is the commit where I admit that having more than one input/output
format for readers and writers was complicating the code too much for a
very small gain, and that it would be easier to only have one way to do
it.

So such way is now:

- Returning (or yielding) a dict if you have key-value type collections.
- Returning (or yielding) a tuple if you have a list-type collection.
- Returning (or yielding) something else otherwise, which will continue
  to work like the old "arg0" format.

IOFORMAT options has been removed in favour of a RemovedOption, which
will complain if you're still trying to set it to anything else than the
one value allowed.

tests/execution/test_node.py

@@ -0,0 +1,104 @@
+from bonobo import Bag, Graph
+from bonobo.strategies import NaiveStrategy
+from bonobo.util.testing import BufferingNodeExecutionContext, BufferingGraphExecutionContext
+
+
+def test_node_string():
+    def f():
+        return 'foo'
+
+    with BufferingNodeExecutionContext(f) as context:
+        context.write_sync(Bag())
+        output = context.get_buffer()
+
+        assert len(output) == 1
+        assert output[0] == (('foo', ), {})
+
+    def g():
+        yield 'foo'
+        yield 'bar'
+
+    with BufferingNodeExecutionContext(g) as context:
+        context.write_sync(Bag())
+        output = context.get_buffer()
+
+        assert len(output) == 2
+        assert output[0] == (('foo', ), {})
+        assert output[1] == (('bar', ), {})
+
+
+def test_node_bytes():
+    def f():
+        return b'foo'
+
+    with BufferingNodeExecutionContext(f) as context:
+        context.write_sync(Bag())
+
+        output = context.get_buffer()
+        assert len(output) == 1
+        assert output[0] == ((b'foo', ), {})
+
+    def g():
+        yield b'foo'
+        yield b'bar'
+
+    with BufferingNodeExecutionContext(g) as context:
+        context.write_sync(Bag())
+        output = context.get_buffer()
+
+        assert len(output) == 2
+        assert output[0] == ((b'foo', ), {})
+        assert output[1] == ((b'bar', ), {})
+
+
+def test_node_dict():
+    def f():
+        return {'id': 1, 'name': 'foo'}
+
+    with BufferingNodeExecutionContext(f) as context:
+        context.write_sync(Bag())
+        output = context.get_buffer()
+
+        assert len(output) == 1
+        assert output[0] == {'id': 1, 'name': 'foo'}
+
+    def g():
+        yield {'id': 1, 'name': 'foo'}
+        yield {'id': 2, 'name': 'bar'}
+
+    with BufferingNodeExecutionContext(g) as context:
+        context.write_sync(Bag())
+        output = context.get_buffer()
+
+        assert len(output) == 2
+        assert output[0] == {'id': 1, 'name': 'foo'}
+        assert output[1] == {'id': 2, 'name': 'bar'}
+
+
+def test_node_dict_chained():
+    strategy = NaiveStrategy(GraphExecutionContextType=BufferingGraphExecutionContext)
+
+    def uppercase_name(**kwargs):
+        return {**kwargs, 'name': kwargs['name'].upper()}
+
+    def f():
+        return {'id': 1, 'name': 'foo'}
+
+    graph = Graph(f, uppercase_name)
+    context = strategy.execute(graph)
+    output = context.get_buffer()
+
+    assert len(output) == 1
+    assert output[0] == {'id': 1, 'name': 'FOO'}
+
+    def g():
+        yield {'id': 1, 'name': 'foo'}
+        yield {'id': 2, 'name': 'bar'}
+
+    graph = Graph(g, uppercase_name)
+    context = strategy.execute(graph)
+    output = context.get_buffer()
+
+    assert len(output) == 2
+    assert output[0] == {'id': 1, 'name': 'FOO'}
+    assert output[1] == {'id': 2, 'name': 'BAR'}