Correct iterator collection and partial-retention benchmarks

[charliermarsh]

Summary

bench_from_iter currently measures conversion from a slice, and bench_retain_mut_half keeps every element because its input is filled with the even value 16.

We now collect a copied iterator in the collection benchmarks and use consecutive integers in the retention benchmarks, so the half-retention cases actually remove half the elements. This applies to both SmallVec and the Vec controls.