Improve bytes per second benchmark output

[software-opal]

I'm building a library that implements a somewhat slow compression algorithm and I wanted to see what sort of speeds I was getting. Using the benchmark I can get the speeds for some tests; but not for the slower ones:

test compress_large::level_0  ... bench:  10,848,110 ns/iter (+/- 343,409) = 1 MB/s
test compress_random::level_0 ... bench: 257,280,250 ns/iter (+/- 19,228,907)

It would be nice to have the speed output in other units, like KB/s and B/s. It could also be extended to produce GB/s if needed.

[JJJollyjim]

I had a go at implementing this: JJJollyjim@f9d2caf

As I understand it, it's a breaking change to an (unstable) API, since it changes the BenchSamples struct which is public in libtest. It could probably be implemented without the breaking by splitting the rate into a whole part (which stays called mb_s), and a fractional part (bytes per second modulo 1,000,000), but I'm not sure if that's worth the confusion.

I left the json output as-is for now, not sure if that's the best approach.

---

~/M/benchmark-bytes-test ❯❯❯ cargo +stage1 bench                                                                                                                                                                                             ✘ 130 
   Compiling benchmark-bytes-test v0.1.0 (/home/jamie/Misc/benchmark-bytes-test)
    Finished release [optimized] target(s) in 0.37s
     Running target/release/deps/benchmark_bytes_test-80ad362b39f8674c

running 5 tests
test tests::bench_50_b  ... bench:  20,217,836 ns/iter (+/- 260,903) = 49 B/s
test tests::bench_50_gb ... bench:     151,659 ns/iter (+/- 2,454) = 39 GB/s
test tests::bench_50_kb ... bench:     151,641 ns/iter (+/- 1,520) = 39 kB/s
test tests::bench_50_mb ... bench:     151,586 ns/iter (+/- 271) = 39 MB/s
test tests::bench_50_tb ... bench:     151,668 ns/iter (+/- 1,658) = 39560 GB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 5 measured; 0 filtered out