Make == for associative arrays test for equality, not identity. _aaEq was added to runtime/internal/aaA.d which forwards to TypeInfo_AssociativeArray.equals in genobj.d. On the codegen side, DtoAAEquals was added to gen/aa.cpp and is called from EqualExp::toElem in gen/toir.cpp. I assume that the frontend will produce an error if == is used on associative arrays of different type. This fixes DMD bug 1429.

struct Vector {
        union { float x; float y; }
        const static Vector zero = { x : 0, y : 0 };
}

struct HBoxLayout {
        Vector padding  = Vector.zero;
}