Upgrade core library version 0.83.0 -> 0.83.1

Bugfix release. For details, see https://github.com/Tightdb/tightdb/blob/master/release_notes.md#0831-release-notes.

I have updated CHANGELOG.txt, but I have only added this:

> Updating to core library version 0.83.1

I'll let you guys decide on what additional details to add.