Moved washers.scad from screws.scad to nuts.scad.

Moved screw_insert() from inserts.scad to screw.scad.
Changed most includes of inserts.scad to use insert.scad.
Removed linear_bearings, iecs, rockers and microswitches from lib.scad as
already included by other vitamins.

printed/box.scad

@@ -35,8 +35,7 @@ include <../core.scad>
 use <../vitamins/sheet.scad>
 use <../vitamins/screw.scad>
 use <../vitamins/washer.scad>
-
-include <../vitamins/inserts.scad>
+use <../vitamins/insert.scad>
 use <../utils/quadrant.scad>
 
 bezel_clearance = 0.2;