Implement entity relationships in ECS

[benreid24]

Resolves #173

Adds the following relationship types in ECS:

• Parenting:
  • Children are removed when parent is removed
  • Includes ParentAware component helper base class
• Dependencies:
  • Defers entity destruction until all users are destroyed

Introduces the IndexMappedList container which uses 0-based indices as keys into a linked list of payloads that are stored in one contiguous region in memory.