flat_map vs other
test nested_iter  ... bench:      4981 ns/iter (+/- 8)
-O3
was:
test nested_iter  ... bench:      1648 ns/iter (+/- 4)
test flatmap_iter ... bench:     15356 ns/iter (+/- 214)
test nested_iter  ... bench:      2365 ns/iter (+/- 10)
test trivec_iter  ... bench:      2692 ns/iter (+/- 5)
-O
test fast_iter           ... bench:      4036 ns/iter (+/- 43)
test flatmap_iter        ... bench:      7580 ns/iter (+/- 129)
test nested_iter         ... bench:       822 ns/iter (+/- 3)
test nested_iter_counter ... bench:      1200 ns/iter (+/- 1)
test nested_unwrap_iter  ... bench:      1219 ns/iter (+/- 2)
test trivec_iter         ... bench:      4368 ns/iter (+/- 6)
-O3
test fast_iter           ... bench:      4020 ns/iter (+/- 42)
test flatmap_iter        ... bench:      7953 ns/iter (+/- 964)
test nested_iter         ... bench:       822 ns/iter (+/- 2)
test nested_iter_counter ... bench:      3893 ns/iter (+/- 444)
test nested_unwrap_iter  ... bench:      3899 ns/iter (+/- 9)
test trivec_iter         ... bench:      4321 ns/iter (+/- 12)
-O3 lto
test fast_iter           ... bench:       964 ns/iter (+/- 14)
test flatmap_iter        ... bench:      5611 ns/iter (+/- 19)
test nested_iter         ... bench:       822 ns/iter (+/- 10)
test nested_iter_counter ... bench:      3896 ns/iter (+/- 7)
test nested_unwrap_iter  ... bench:      3866 ns/iter (+/- 471)
test trivec_iter         ... bench:      4362 ns/iter (+/- 9)
asm: Update