Mark some weapons as requiring a prerequisite - only able to buy if the prerequisite has been purchased.

• Add "Prerequisite" field to gun JSON
• In equip menu, only show the most upgraded version
  • Instead of sell, show "downgrade"
  • When an upgrade is available, show "upgrade" instead of "buy"
• When picking up a gun upgrade, automatically give the player all the prerequisites, and replace the prerequisite with the upgrade if they are holding it

Use for:

• Pistol -> akimbo pistol
• In Sand, khanjali -> wurmtooth; can also add a laser pistol -> lasgun
• In Doom, pistol -> chaingun, fists -> chainsaw
• Cyberdogs chainsaws

Requires #739