Refactor `util._extend` to `Object.assign`

[benjamingr]

Hey, I figured we can change util._extend to Object.assign in various places and replace util._extend itself with Object.assign.

As this is a small (but breaking) change for edge cases I figured I'd bring it up for discussion first.

The only difference is that assign copies symbols as well - I don't think it'll cause any issues.

[cjihrig]

See #4593

[benjamingr]

Thanks! Looks like the discussion is already happening there.