Export some Squirrel API for extension modules
vastspace: Allow adding as many Firearm types as possible via script

This is first attempt to implement framework to create multiple classes
by Squirrel script only.  As a demonstration, M16 assault rifle and M40
sniper rifle use the same C++ class but different static parameters.

This mechanism is similar to variants for Destroyer, but it only
provided means to define default turret types.

Introducing this mechanism actually reduces line of codes by removing
redundant codes in C++.
vastspace: Make HUD overlay icons customizable for Firearms

Now, every aspect of a Firearm can be customized without native code.