Add slic3r, tool used in 3d printing

This package Adds slic3r, a tool to generate movement commands for 3d printers from STL files. 

There are two things to note: It's a prerelease, as the current release causes problems with NixOS' perlPackages when using an older perl. Because it uses a newer perl, it displays a warning when running: (```Running Slic3r under Perl >= 5.16 is not supported nor recommended```). 

I ignored this error for the last two weeks and never noticed a problem when printing objects.