chore(iterators): return slice of key instead of a vec

[rymnc]

reference: #2092 (comment)

As per @xgreenx's comment, we should investigate if there is a clean way to return a slice of the key instead of an owned vec

[xgreenx]

I think it is not possible since we call next on the iterator that requires &mut. Plus I assume that underlying iterator will deallocate the memory, so we can close this issue=)