[WIP] Convert save system to json.h.

I actually have most of the load/save system converted to using this, will push it up when i finish testing.

I might change the `read_into(var)` methods to just `read(var)`, and move them to JsonIn for symmetry with `JsonOut::write()`.

The main thing missing is automatic de/serialization to/from maps. It should be easy to add once the syntax settles down.

For examples of how automatic serialization works for cata data structures, check out https://github.com/yobbobanana/Cataclysm-DDA/commit/ccfcce471b50653369b4b218eee6109102dd8b45 and https://github.com/yobbobanana/Cataclysm-DDA/commit/33c033f70c3d30e4dbcc1d512f1be1519f9b2af9

Those two throw errors on failure (which is by far my preference), but can be easily be changed to fail silently.