more icons

[mkaput]

New icons for build.rs, mod.rs & main.rs/lib.rs. I had no idea what symbol to choose for two former ones, so I opted for colouristic variants.

I have removed svgs in favour of Illustrator files, because this format is easier to modify

fix #1038

[matklad]

Super!

[matklad]

We might want to think about replacing colors with different icons (like in the build.rs case), to help colorblind people. And I'll take a look if we can easily move mod.rs to the top.

[matklad]

29d42e6

[matklad]

[matklad]

And we might want to look at something like NestingTreeStructureProvider, to really display the module structure instead of file structure.

Though I'd prefer to stick with the current solution: it's less magic there.

[johnthagen]

Just want to say this is one of those features I didn't know I needed until it was added. Great job! 👍