[config] Refactoring of configurables, allowing partially configured objects.

Configurables did not allow more than one "method" option, and mixed
scenarios (options+methods+...) were sometimes flaky, forcing the user
to know what order was the right one. Now, all options work the same,
sharing the same "order" namespace.

Backward incompatible change: Options are now required by default,
unless a default is provided.

Also adds a few candies for debugging/testing, found in the
bonobo.util.inspect module.

tests/config/test_methods_partial.py

@@ -0,0 +1,66 @@
+from unittest.mock import MagicMock
+
+from bonobo.config import Configurable, ContextProcessor, Method, Option
+from bonobo.util.inspect import inspect_node
+
+
+class Bobby(Configurable):
+    handler = Method()
+    handler2 = Method()
+    foo = Option(positional=True)
+    bar = Option(required=False)
+
+    @ContextProcessor
+    def think(self, context):
+        yield 'different'
+
+    def call(self, think, *args, **kwargs):
+        self.handler('1', *args, **kwargs)
+        self.handler2('2', *args, **kwargs)
+
+
+def test_partial():
+    C = Bobby
+
+    # inspect the configurable class
+    with inspect_node(C) as ci:
+        assert ci.type == Bobby
+        assert not ci.instance
+        assert len(ci.options) == 4
+        assert len(ci.processors) == 1
+        assert not ci.partial
+
+    # instanciate a partial instance ...
+    f1 = MagicMock()
+    C = C(f1)
+
+    with inspect_node(C) as ci:
+        assert ci.type == Bobby
+        assert not ci.instance
+        assert len(ci.options) == 4
+        assert len(ci.processors) == 1
+        assert ci.partial
+        assert ci.partial[0] == (f1,)
+        assert not len(ci.partial[1])
+
+    # instanciate a more complete partial instance ...
+    f2 = MagicMock()
+    C = C(f2)
+
+    with inspect_node(C) as ci:
+        assert ci.type == Bobby
+        assert not ci.instance
+        assert len(ci.options) == 4
+        assert len(ci.processors) == 1
+        assert ci.partial
+        assert ci.partial[0] == (f1, f2,)
+        assert not len(ci.partial[1])
+
+    c = C('foo')
+
+    with inspect_node(c) as ci:
+        assert ci.type == Bobby
+        assert ci.instance
+        assert len(ci.options) == 4
+        assert len(ci.processors) == 1
+        assert not ci.partial