Collector refactoring, part 1

[Kobzol]

This is a series of commits that refactor the collector crate in order to make it easier to use it as a library, remove unused code, better structure code to break up large files and to make future integration of runtime benchmarks easier. I will continue with other parts after (if) this gets merged.

Best reviewed commit by commit.

[nnethercote]

This mostly seems fine. One suggestion: anything called utils typically ends up as a dumping ground for all sorts of stuff, and might as well be called junk_drawer. Does read2 need to be moved? Could fs be a module at the top-level, next to read2?

[Kobzol]

I don't like utils.rs files that contain a lot of unrelated things, but I often create utils modules with a file per area of responsibility. A stylistic choice I guess :)

Here I have done it mostly to orient myself in the code. The root contained several files that were intertwined a lot (some of them binaries, some of them included from main.rs, some of them included from lib.rs), so I wanted to group the files into directories to make sense of it. If you don't mind, I would keep it like this until I finish the refactoring, then if you find it clearer I can put the files back into the root.

[Kobzol]

Rebased on master.