Provide private symbols through internal APIs

Adds a notion of private symbols, mainly intended for internal use, especially, self-hosting of built-in types that would otherwise require new C++ classes.

On the JS side (i.e., in built-ins), private properties can be created and accessed through a set of macros:

  NEW_PRIVATE(print_name)
  HAS_PRIVATE(obj, sym)
  GET_PRIVATE(obj, sym)
  SET_PRIVATE(obj, sym, val)
  DELETE_PRIVATE(obj, sym)

In the V8 API, they are accessible via a new class Private, and respective HasPrivate/Get/Private/SetPrivate/DeletePrivate methods on calss Object.

These APIs are designed and restricted such that their implementation can later be replaced by whatever ES7+ will officially provide.

R=yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/48923002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17683 ce2b1a6d-e550-0410-aec6-3dcde31c8c00