There were two functions screw-insert-holes and screw-insert-screw-holes
whose difference was unclear. screw-insert-screw-holes creates the holes
for the bottom plate only, so it was renamed to
screw-insert-holes-plate.

Responsibility for creating and positioning shapes of screw insert
holes was scattered across multiple methods which made it hard to
maintain. In particular, dimensions of the screw insert shape had to
be passed to the `screw-insert` and `screw-placement` functions which
do not need this information, as they mostly do positioning work.
Screw insert shapes are now created directly in `screw-insert-hole`
and others. `screw-insert` is renamed to `screw-placement-common` to
better reflect the changes.

Bottom plate generation in manuform.clj relied on several hacky 3D
operations (micro-translations before projection operations and
extruded screw hole shapes). This is replaced with 2D operations
followed by a single linear_extrude. This commit also fixes the
non-aligning bottom plate mentioned in ibnuda#112.