Collaboration tools (rustfmt, clippy)

After my last PR I noticed you had to clean up a bit. That made me wonder if it makes sense to configure and use `clippy` and `rustfmt`:

* Since `clippy` is probably less controversial I went ahead and addressed all current issues, either by changing code (where I thought `clippy` made sense), or disabling lints (e.g., where faster bends rules for speed). Some of the more pedantic lints could be discussed, e.g., usage and formatting of number literals (`unreadable_literal`, `unseparated_literal_suffix`). https://github.com/ralfbiedert/faster/tree/clippy

* I think `rustfmt` makes sense as well, but needs more configuration to resonate with the code. I found a few settings that worked for me (e.g., `max_width` be set rather high not to break up most macros which makes them harder to read). However you should probably take the lead on that one.

Let me know what you think about clippy, in particular `unreadable_literal`, `unseparated_literal_suffix` and `type_complexity` (I prefer the former 2, no opinion on 3rd). I can then create another PR.