Began updating the game saves to nested hash maps

This will allow for entirely arbitrary changes to the statistics data
structures in the future (such as from adding a new mode or difficulty).
Finished planning the data changes

Added the needed vars, work should be completed for GameStats.cs in the
next commit
Updated how stats are stored to be modular

Arbitrary new modes, difficulties, and levels can be added without
breaking stats.

Work is needed to get this added code entirely up and running, but it is
pretty close to done.
Fixed empty instantiation bug

The level data list's elements were being left null.

This was due to a bug I had created assuming that the default list
constructor instantiated a default value for list locations, rather than
just allocating capacity.

The revised level stats for in-build levels now works (needs cleaning
up). Custom level stats are next for revision.