`FlatMap` can use internal iteration for its `fold`, which shows a
performance advantage in the new benchmarks:

    test iter::bench_flat_map_chain_ref_sum ... bench:   4,354,111 ns/iter (+/- 108,871)
    test iter::bench_flat_map_chain_sum     ... bench:     468,167 ns/iter (+/- 2,274)
    test iter::bench_flat_map_ref_sum       ... bench:     449,616 ns/iter (+/- 6,257)
    test iter::bench_flat_map_sum           ... bench:     348,010 ns/iter (+/- 1,227)

... where the "ref" benches are using `by_ref()` that isn't optimized.
So this change shows a decent advantage on its own, but much more when
combined with a `chain` iterator that also optimizes `fold`.