Depth first execution in sync mode

[sunu]

fixes #165

[pudo]

Upon consideration, I'd like to make the argument against this change:

• It introduces a whole different execution model that kicks in sometimes, which makes it even harder to imagine what memorious does and how.
• It's not clear how this is different/better than just calling the sub-method from the iterator one. It seems to just create a really convoluted mechanism for method invocation.
• It's not clear to me what this does to crawlers that actually assume that recursion is the right way to do iteration in memorious. Will they still work?

So, arguably, we're introducing a whole new order of magnitude of complexity to enable something that's pretty smelly behaviour to begin with (generating a million tasks in memory).