Add something like Ash's Node structure

[Hrothen]

In the Ash framework Richard Lord uses a structure he calls a node that holds references to multiple components, systems then access these nodes instead of the components directly. Using nodes as the default behavior for Skald would conflict with the goal of providing code that is easily optimized by the compiler, but providing end users access to something similar would allow them to write very concise code in places where they don't need to worry as much about maximizing speed.