add throughput to benches

[TheIronBorn]

Throughput often provides more precision when the benchmarks take a couple of nanoseconds

for example, there's a difference between these two hidden just by the ns/iter:

test bench_fx64_003    ... bench:           2 ns/iter (+/- 1) = 1500 MB/s
test bench_fx64_004    ... bench:           2 ns/iter (+/- 0) = 2000 MB/s