Consider free function to access dims instead of member functions

When accessing a dim of a shape, ideally we would allow simply `auto x = s.dim<0>()`. Unfortunately, the reality of template syntax issues in many situations means it has to be `auto x = s.template dim<0>()`.

This is the main reason I added named aliases like `auto x = s.x()`. 

But, maybe a better direction to go is a free function `dim<i>(s)`. 