Completely rewrite everything.

Massively slimmed down. The approach before was too concerned with
making everything intimately aware that the map was chunked when the
only important thing is that the Map provides access to the contents
inside.

Also means we don't have to provide the content class.
Update the unit tests to match new API
Don't need a JSON library currently
Code layout

Move the indexer up with the other properties.
Rename T to U for ChunkedMap.Chunk<> to avoid clash
Fix mistake on Within(...)'s loop logic

Accidentally an infinite loop.
Scope the existing chunk in GetChunk to avoid name clashing.
Fix a dumb error in getting a new chunk.

Creating a new chunk, then adding another new chunk to the collection
then returning the chunk that only exists on the stack is not the best
way to go for retaining changes.
Change indexers set logic

Modulus, not division, 1/16 is 0, 1%16 is 1. Oops.