Override Cycle::try_fold

It's not very pretty, but I believe this is the simplest way to correctly implement `Cycle::try_fold`. The following may seem correct:
```rust
loop {
    acc = self.iter.try_fold(acc, &mut f)?;
    self.iter = self.orig.clone();
}
```
...but this loops infinitely in case `self.orig` is empty, as opposed to returning `acc`. So we first have to fully iterate `self.orig` to check whether it is empty or not, and before _that_, we have to iterate the remaining elements of `self.iter`.

This should always call `self.orig.clone()` the same amount of times as repeated `next()` calls would.

r? @scottmcm