E2020t and E4040t thinner extrusions added to work with the brackets.
Extrusions can now have recessed channels, round or square centre sections and holes.
Fixed the shape of extrusion centre section spars.
T-nuts now have 45 degree chamfers instead of a fixed 1mm minimum thickness.
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.
