Interpolate arbitrary Iterator multiple times

Interpolation does support arbitrary Iterators, but they only can be used once. The types that can be used multiple times seem to be hardcoded in [src/runtime.rs](https://github.com/dtolnay/quote/blob/master/src/runtime.rs) (`[T]`, `Vec`, `BTreeSet`). References and `RepInterp` do not seem useful for this case because Iterators usually aren't `Copy`.

Possible fixes:
- Call clone on the Iterator if it is used multiple times.
- Support arbitrary IntoIterator, which can be used multiple times idiomatically. This could also replace the special casing for `[T]`, `Vec`, and `BTreeSet`.