External type support at compile time

[randy408]

In conjunction with ECS_ALIAS() it would allow typedefs + meta aliases of external types.

ECS_STRUCT_EXTERN(vec3,
{
    float x;
    float y;
    float z;
});

ECS_ALIAS(vec3, Position); //meta alias and typedef

ECS_STRUCT(example,
{
    Position p;
    bool b;
});

//everything is registered with the same macro
ECS_META(world, vec3);
ECS_META(world, Position);
ECS_META(world, example);