ES6 symbols: implement name property

Adds string-valued name property to symbols, and uses it for pretty-printing.

Requires allocating symbols in pointer space, with a custom iterator to skip the unboxed hash.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/12459026

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14053 ce2b1a6d-e550-0410-aec6-3dcde31c8c00