Create bench_cmp command

[s7tya]

I've implemented a small example command for comparing two artifacts. Currently, it only displays the range, mean, and count across all series. This is the first step towards #1734. I'm not sure if the way I'm collecting data is correct or not.

$ cargo +nightly run --release --bin collector bench_cmp a b

+--------+-------+------------------+--------+
| name   | count | range            | mean   |
+--------+-------+------------------+--------+
| ❌     | 43    | [-0.50%, -0.20%] | -0.28% |
+--------+-------+------------------+--------+
| ✅     | 68    | [+0.20%, +0.73%] | +0.32% |
+--------+-------+------------------+--------+
| ✅, ❌ | 111   | [-0.50%, +0.73%] | +0.09% |
+--------+-------+------------------+--------+

[Kobzol]

Thank you for working on this! :) I would like to avoid one big PR that will implement some super-duper UI at once, so let's first land the basic CLI interface, and just output a single useful piece of information (which compiler is faster on instruction counts on average, and by how much). Then you can improve the UI incrementally piece by piece.

[Kobzol]

To clarify: even though I suggested just to output a simple text line for simplicity, you can keep the table that you have right now, if you want. Although I suppose that soon it will be replaced by a more complex UI anyway.

[s7tya]

Thanks! I've fixed the requested changes.

[Kobzol]

It's really cool to be able to just run a CLI command and instantly see the difference :) This is gonna be very useful. Left a few more comments.

[Kobzol]

Awesome. Tried it locally and it works great. Thank you!