Upgrade Symbol implementation to match current ES6 behavior.

Refresh the implementation of Symbols to catch up with what the
specification now mandates:

* The global Symbol() function manufactures new Symbol values,
  optionally with a string description attached.

* Invoking Symbol() as a constructor will now throw.

* ToString() over Symbol values still throws, and
  Object.prototype.toString() stringifies like before.

* A Symbol value is wrapped in a Symbol object either implicitly if
  it is the receiver, or explicitly done via Object(symbolValue) or
  (new Object(symbolValue).)

* The Symbol.prototype.toString() method no longer throws on Symbol
  wrapper objects (nor Symbol values.) Ditto for Symbol.prototype.valueOf().

* Symbol.prototype.toString() stringifies as "Symbol("<description>"),
  valueOf() returns the wrapper's Symbol value.

* ToPrimitive() over Symbol wrapper objects now throws.

Overall, this provides a stricter separation between Symbol values and
wrapper objects than before, and the explicit fetching out of the
description (nee name) via the "name" property is no longer supported
(by the spec nor the implementation.)

Adjusted existing Symbol test files to fit current, adding some extra
tests for new/changed behavior.

LOG=N
R=arv@chromium.org, rossberg@chromium.org, arv, rossberg
BUG=v8:3053

Review URL: https://codereview.chromium.org/118553003

Patch from Sigbjorn Finne <sigbjornf@opera.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19490 ce2b1a6d-e550-0410-aec6-3dcde31c8c00