Fixed C++ union constructor violation in float2/float4/Box2D

[jstine35]

This PR is of particular interest! Review the changes I made to Box2D. It might look like alien or unfamiliar syntax at first. C++14's implicit type deduction for {} initializer lists is kind of an amazingly powerful tool.

Notes have been added to the unions explaining why they intentionally do not have constructors, and the intention to use C++14's very nifty and incredibly powerful initializer-list syntax in place of the redtape-laden constructors that also break handy union things.

This PR fixes an error which shows up in clang/gcc when you try to nest non-trivial unions inside of other unions. Visual C++ ignores the error even though it's clearly forbidden per C++ Standard Rules.

[jstine35]

@robinlavallee

./h/x-simd.h:589:17: error: member ‘float2 float4::<unnamed struct>::xy’ with constructor not allowed in anonymous aggregate
         float2  xy,zw;
                 ^~
./h/x-simd.h:589:20: error: member ‘float2 float4::<unnamed struct>::zw’ with constructor not allowed in anonymous aggregate
         float2  xy,zw;
                    ^~

[jstine35]

Use of the union itself inside another union -- in this particular case -- is a convenience factor, and it could be accomplished other ways. The advantage of this layout is that it mimics gpu pixel/vertex shader syntax and behavior.