Initialize submap values on construction.
Convert submap save/load to use json for backwards compatability.
Silence signed vs unsigned comparison warnings.
Add a version member to each submap and bump the save version.
Stop writing out keyfile.
Move legacy map loading methods to savegame_legacy and pull file name generation into unserialize_submaps.
Push file opening down into save_quad().
Try and reduce cross dependencies a little.
Merge pull request #7072 from kevingranade/json-saves

[READY] Json map saves