util: deprecate util._extend
Mark util._extend as deprecated and replace all internal usage with the language provided Object.assign.
util._extend is a long standing member of the undocumented and unofficial public API. It has never been removed because, quite frankly, it is ridiculously useful and is used extensively throughout node core itself.
With the addition of support for Object.assign in node, and the use of other ES6isms, there is now an alternative to maintaining util._extend and adding it to the API docs.
/cc @sam-github @jasnell