Can't have single use renderables

[Nemo157]

Since Render takes itself by reference anything that implements it needs to support re-rendering multiple times. While working on supporting modified event streams in (second example at maud-pulldown-cmark) I found this limiting as pulldown_cmark::Parser isn't reusable, requiring me to wrap stuff up in closures to allow re-creating the parser when necessary. In most cases it's very unlikely that these structs would be kept around, so I was wondering if it were possible to have fn render(self, w: &mut Write) -> fmt::Result and provide implementations for references where needed.

[Nemo157]

To see whether this would work I attempted to implement it (https://git.nemo157.com/forks/maud/commit/f1ecd07). Took a while to work out how to get it working, but finally realised that emulating the Fn, FnOnce hierarchy would work, so in my code I could just impl RenderOnce and consume the Parser while rendering. If you want this I could open a pull request (+ RenderMut maybe, might be useful for some other advanced renderable?).

[Nemo157]

I threw together an implementation of RenderMut and some better tests for both (64565e8).

[lambda-fairy]

Cheers for the report. I agree that having a way to render a value by-move would be useful. But I'm hesitant about making that the default. After all, println!("{}", x) borrows x rather than moving it.

As for RenderMut: do you have a use case for this trait? I don't want to add it in unless there's a real use for it.

[Nemo157]

I can't think of any real use case for RenderMut. I just remembered that it will still default to borrowing a value if it implements Render because of the existence of impl<'a, T: Render> RenderOnce for &'a T and calling the method via use ::maud::RenderOnce; $expr.render(&mut *$w). The compiler will auto-ref $expr till it finds the implementation of RenderOnce for &$expr then call that. I'll add a test that shows that in my changes for #26.