V3: Removed indirection from asset graph

[alshdavid]

• AssetNode and DependencyNode stored directly in the AssetGraph avoiding the need for external Vec<_> and tracking both the node_index and vec_index
• AssetGraph only handle data storage and operation on the data structure are externalized and done at the call site.
• Using **_nx naming convention for AssetGraphNodes, eg asset_nx (meaning asset_node_index)

[alshdavid]

Separated this into a dedicated function, removed recursion and removed the closure. I removed the closure because it's annoying to deal with move semantics in the closure - particularly as we refactor the request tracker approach in the future.

[alshdavid]

This is more a concern for the binding layer rather than of the AssetGraph itself. Extracted it into a function, should probably move it into node_bindings

[alshdavid]

Trying to keep this data structure focused on being a holder of data and minimize manipulation of that data in methods.

Essentially, this is just a graph with add/get methods, consumers decide what to do with the data so the details of manipulations to the data live at the call site