Reduce code duplication for Vector-like classes

[TzuChieh]

WIP: vector2 is now using CRTP base; test for more scenes and other platforms; do this to other vec types if everything looks good

Add a Vector4 type, use CRTP for actual return type (most derived); by default methods are protected, use using declaration to expose

[TzuChieh]

Done. be0c1169637c2ed7d703aa2583d045ff77a3162d