Halve all armour values

This seems like a good start. At least now when I put on light survivour gear and stand 1 tile away from a turret, I actually die after getting shot a dozen times.
Update layering

Make layering penalties consistent with master branch for testing purposes.
bugfix

I HATE SEMICOLONS
Prevent cutting up of items that are made up of materials (one or more parts) that couldn't be cut separately. Near term fix for "I cut my shotgun into 16 skewers."
Merge pull request #7773 from Izicata/patch-4

Halve all armour values
Handle sets of material vs. just returning one material.
Merge remote-tracking branch 'upstream/master' into issue7508_some_fixes_for_item_cutting