Implicit coercion of Symbols

[Domenic Denicola]

I re-read through this whole thread and realized nobody brought up the fact that this *specific* change, of making `String(symbol)` work while `symbol+""` throws, was discussed and agreed upon previously:

- https://esdiscuss.org/topic/string-symbol
- https://github.com/tc39/tc39-notes/blob/master/es6/2014-09/sept-23.md#41-spec-status-report

I realize people are presumably having second thoughts, but I thought it'd be worth linking to the previous thread for anyone who hasn't seen it and thinks this is a new debate.