[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_pickle.py

@@ -2,10 +2,9 @@ import pickle
 
 import pytest
 
-from bonobo import Bag, PickleReader, PickleWriter, settings
-from bonobo.constants import BEGIN, END
+from bonobo import Bag, PickleReader, PickleWriter
 from bonobo.execution.node import NodeExecutionContext
-from bonobo.util.testing import CapturingNodeExecutionContext, FilesystemTester
+from bonobo.util.testing import BufferingNodeExecutionContext, FilesystemTester
 
 pickle_tester = FilesystemTester('pkl', mode='wb')
 pickle_tester.input_data = pickle.dumps([['a', 'b', 'c'], ['a foo', 'b foo', 'c foo'], ['a bar', 'b bar', 'c bar']])
@@ -14,10 +13,8 @@ pickle_tester.input_data = pickle.dumps([['a', 'b', 'c'], ['a foo', 'b foo', 'c
 def test_write_pickled_dict_to_file(tmpdir):
     fs, filename, services = pickle_tester.get_services_for_writer(tmpdir)
 
-    with NodeExecutionContext(PickleWriter(filename, ioformat=settings.IOFORMAT_ARG0), services=services) as context:
-        context.write(BEGIN, Bag({'foo': 'bar'}), Bag({'foo': 'baz', 'ignore': 'this'}), END)
-        context.step()
-        context.step()
+    with NodeExecutionContext(PickleWriter(filename), services=services) as context:
+        context.write_sync(Bag({'foo': 'bar'}), Bag({'foo': 'baz', 'ignore': 'this'}))
 
     with fs.open(filename, 'rb') as fp:
         assert pickle.loads(fp.read()) == {'foo': 'bar'}
@@ -29,25 +26,17 @@ def test_write_pickled_dict_to_file(tmpdir):
 def test_read_pickled_list_from_file(tmpdir):
     fs, filename, services = pickle_tester.get_services_for_reader(tmpdir)
 
-    with CapturingNodeExecutionContext(
-        PickleReader(filename, ioformat=settings.IOFORMAT_ARG0), services=services
-    ) as context:
-        context.write(BEGIN, Bag(), END)
-        context.step()
+    with BufferingNodeExecutionContext(PickleReader(filename), services=services) as context:
+        context.write_sync(Bag())
+        output = context.get_buffer()
 
-    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] == {
+    assert len(output) == 2
+    assert output[0] == {
         'a': 'a foo',
         'b': 'b foo',
         'c': 'c foo',
     }
-    assert args1[0].args[0] == {
+    assert output[1] == {
         'a': 'a bar',
         'b': 'b bar',
         'c': 'c bar',