Interpolate arbitrary Iterator multiple times

[lukaslihotzki]

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 ([T], Vec<T>, BTreeSet<T>). 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<T>, and BTreeSet<T>.