Finish `Sparse` Data Structure APIs

[Behemyth]

The sparse group of data structures including SparseHashMap are only minimally implemented to facilitate a minimal EntityRegistry. In addition to being barely functional, there is opportunity for code reuse between the different Sparse implementation varieties.

All data structures should mimic their corresponding STL interface except with additional functionality. User functions follow Synodic Software convention while shared functionality with STL like begin size_type etc follow STL convention.