Make possible to use one-shot iterators as children nodes.

[astynax]

For now we cannot use iterators like itertools.chain as child nodes, because _validate_children exhaust such iterators just before the parent node construction. The result looks like original sequences were empty. Yes, user could apply list() such iterators but then the code would look pretty noisy.

I propose to "materialise" as lists such iterators, because we would reuse them anyway during the HTML rendering. We can use the itertools.tee and fork each Iterator before validation. Though, such forking would mean the same materialisation under the hood, but at the same time the code would look a way more complicated.

[pelme]

hi @astynax , thanks a lot for digging into this. Based on your findings I opened #72 . I took a slightly alternative approach of not materialising as lists in the validation. I am afraid that it will break streaming if the evaluation happens directly when the elements are constructed.

[astynax]

@pelme just saw your PR. Your decision to stay lazy during the validation is great! It always good to be able to decide when we want to be lazy and when we should be eager (and force lazy stuff with list() before wrapping it in HTML).

[pelme]

Fixed in #72.