v0.6.4

This is mostly an internal cleanup release, intended to make the codebase easier to work with and make it easier for other modders to write integrations. The only player-facing change is that it is now slightly easier to unlock higher-tier elemental upgrades for your weapons.

• Change: elemental upgrade prerequisites are now relaxed; basic and intermediate
  upgrades need to be level 2 to unlock the next tier, but higher-tier upgrades
  can now be leveled up to match the tier below.
• Change: WeaponInfo objects can now be rebound to arbitrary weapons even of
  different classes; this has no user-facing effect but may be useful for mod
  integrations.
• Change: internal cleanup of Legendoom integration code.
• Change: redesign of GetInfoFor* API family.
• Change: more documentation on modding/addons/integrations.