Code reshuffling and more fleshing out of core types
Despgraph object itself - Noted some speculative features it needs to have

Namely, some "instance_data" datastore, which is used to store evaluation
state/context info which is used as part of multithreaded/threadsafe
evaluation of subgraphs...
Stub for ID-Group node - Nothing here yet...
Atomic Operation Node - First Pass

This is the first attempt a creating a "AtomicOperation" inner node type.
* The arguments used for the callback are still undecided
* Currently it uses an RNA Pointer to refer to the data that's being used,
  which will later be used helping to figure out what the node is for...
General tweaks to make the purpose of the depsgraph_types.h file's purpose clearer
Placeholder stubs in file for core depsgraph stuff
Sketch of datatypes for holding evaluation state/defining node types...