State file: pickle to json

[pchakraborty]

Consider switching to json serialization format for storing the state from the current pickle format. This would ensure that issues like #279 don't come up again. A json.dump without indentation should reduce the risk of hand editing of the state file.

For an older clone, made with mepo 1, a warning will be displayed, along with a solution of how to update to mepo 2 style state

Converting mepo1 state to mepo2 state
Run mepo update-state to permanently convert to mepo2 state

[mathomp4]

Plus we'd be able to read the state file!

That said, could we somehow "write-protect" the file? That is, only "easily" allow changes via mepo? I'd be afraid someone would see a json file and think, oh, I can just edit this.

I mean, it's not hard to chmod +w but at if the script would be able to toggle the write bits, that might make things a bit safer

[pchakraborty]

@mathomp4 That, I think, can be easily done.

We can also consider taking the string generated by json.dumps and convert it to binary via ord or bytearray save that to file. I'll look into this.

[pchakraborty]

The state file is now in json format and is read-only.