Compact E3D v6 2-in-1 fan duct

Idea from https://www.thingiverse.com/thing:839620 but it's a totally new design.

Reuse original E3D v6 fan dust. Place the fan nearly vertically to save space and avoid hitting towers on Kossel Mini printer.

You need M3 x 12mm or 14mm screw to fix the fan. Too long or too short doesn't work. Or you can adjust `fan_fixer_d` in scad model to fit the length of screws you have.

Change `bolt_pillar_lenght` to fit the screws which are used to fix original E3D v6 fan dust.