Add craftable glass shiv.

Take a glass shard, and wrap it in duct tape or a rag. It's still a terrible weapon but at least it won't hurt your hand just by using it.

Also, is it just me, or is the formula for shattering a glass weapon kind of silly?

```C++
if (weapon.made_of("glass") &&
     rng(0, weapon.volume() + 8) < weapon.volume() + str_cur) {
  ...
}
```

Essentially, bigger-volume glass weapons are harder to break. This doesn't seem to make any sense, since a bigger piece of glass can have greater forces act on it (levers, momentum, etc - trying to snap a small piece of glass is harder than a bigger one).

It should probably be based on the thickness, but only armor has the `material_thickness` flag. Perhaps some function of volume+weight? There's a huge difference between smashing someone with a hollow sphere of glass (say, a flask from a chemistry lab) and a solid sphere of glass (say, a crystal ball, or some kind of ornament), even if they have the exact same size.