minimal customizable linkage arm segment

These customizable parts should be usable as building blocks
to quickly experiment with various linkage designs.

e.g. the alternative form of paucellier lipkin linkage like presented in this YT video: watch?v=62QmuQdoBRk

* use 3mm filament as axles
* weld or glue axles in place (make clean rivets with washers)
* stack multiple identical linkages to get more sturdyness
* crop all arms to zero length to get trivial washers