add direct file i/o and possibly load/save to API

[eronoobos]

Shard native can already use standard lua io. functions, but Shard Spring Lua can't. There's nothing I really need this for at the moment, but it seems like something the Shard API should have. AKA it's low-priority.

Load/save would take variables or a table and save them as lua code.

It might be a good or a bad idea to build the Shard version into the filenames, so that there aren't issues with old data being loaded. For the same reason, if it's possible to get a checksum of the game configuration, that could be used to avoid the need to delete outdated files.

[tomjn]

This is problematic as it means on the next game, some shards will have differing contents on the filesystem, and behave differently, and since LuaAIs run synced, it'd cause a game desync