[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/io/test_json.py

@@ -3,21 +3,20 @@ import pytest
 from bonobo import Bag, JsonReader, JsonWriter, settings
 from bonobo.constants import BEGIN, END
 from bonobo.execution.node import NodeExecutionContext
-from bonobo.util.testing import CapturingNodeExecutionContext, FilesystemTester
+from bonobo.util.testing import FilesystemTester
 
 json_tester = FilesystemTester('json')
 json_tester.input_data = '''[{"x": "foo"},{"x": "bar"}]'''
 
 
-def test_write_json_arg0(tmpdir):
+def test_write_json_ioformat_arg0(tmpdir):
     fs, filename, services = json_tester.get_services_for_writer(tmpdir)
 
-    with NodeExecutionContext(JsonWriter(filename, ioformat=settings.IOFORMAT_ARG0), services=services) as context:
-        context.write(BEGIN, Bag({'foo': 'bar'}), END)
-        context.step()
+    with pytest.raises(ValueError):
+        JsonWriter(filename, ioformat=settings.IOFORMAT_ARG0)
 
-    with fs.open(filename) as fp:
-        assert fp.read() == '[{"foo": "bar"}]'
+    with pytest.raises(ValueError):
+        JsonReader(filename, ioformat=settings.IOFORMAT_ARG0),
 
 
 @pytest.mark.parametrize('add_kwargs', ({}, {
@@ -32,24 +31,3 @@ def test_write_json_kwargs(tmpdir, add_kwargs):
 
     with fs.open(filename) as fp:
         assert fp.read() == '[{"foo": "bar"}]'
-
-
-def test_read_json_arg0(tmpdir):
-    fs, filename, services = json_tester.get_services_for_reader(tmpdir)
-
-    with CapturingNodeExecutionContext(
-        JsonReader(filename, ioformat=settings.IOFORMAT_ARG0),
-        services=services,
-    ) as context:
-        context.write(BEGIN, Bag(), END)
-        context.step()
-
-    assert len(context.send.mock_calls) == 2
-
-    args0, kwargs0 = context.send.call_args_list[0]
-    assert len(args0) == 1 and not len(kwargs0)
-    args1, kwargs1 = context.send.call_args_list[1]
-    assert len(args1) == 1 and not len(kwargs1)
-
-    assert args0[0].args[0] == {'x': 'foo'}
-    assert args1[0].args[0] == {'x': 'bar'}