Change LUA_NUMBER to fixed_t, change angle_t handling in Lua.

Angles now go from 0 to 0xFFFF (360 degrees == FRACUNIT) instead
of using a full UINT32. Lua only has one number type, so signedness
gets in the way of using angle_t directly. This handling of angles
matches up with how ZDoom ACS scripting and the like does it.

I also changed all the integer casts and pushes of fixed_t to
their own macro in preperation for possible future seperation.