Upgrade core library version 0.81.0 -> 0.82.0

This release of core makes changes to the `Group` API and adds new function Group::remove_table() and `Group::rename_table()`. For further details, see https://github.com/Tightdb/tightdb/blob/master/release_notes.md#0820-release-notes.

I have updated CHANGELOG.txt, but I have only added this:

    Updating to core library version 0.82.0

I'll let you guys decide on what additional details to add.

Note that core version 0.82.0 requires https://github.com/realm/realm-cocoa/pull/785.

Would word:

Significant performance improvements when holding onto large numbers of RLMObjects.