Add lens entity and recipe for detonator

[byorgey]

This PR adds:

• A new entity called lens
  • Recipe: 1 glass + 1 mithril with a required drill (idea: you grind the piece of glass using a mithril-coated drill bit)
  • For now it does not convey any capabilities, but it seems ripe for some sort of pun on lenses.
• A recipe for detonator:
  • 1 string + 10 curry + 1 lens
  • Idea: curry is explosive, so you attach a string as a fuse and then ignite it by focusing sunlight with the lens.

Because of the mithril, these are not so easy to craft (Level 13 and 14, respectively) but I don't know any particular reason that these need to be easy to craft. If we wanted, we could easily remove the mithril, which would result in them being Level 6 and 7 instead.

Closes #1017.