Memoize polyglot instance during render

Finally! I found some time to push a long time request. We're leaving behind the old way to make way for new ones:

• Stop using componentWillReceiveProps and use memoization instead 📝
• A bunch of package updates 🆙
• The baseline required is now react@16.8.0 due to the use of useMemo. If you cannot upgrade, please use the latest version below this i.e. v0.6.1.

Thanks a bunch to @soda0289 for this contribution and all who participated.