Remove sliced string string type

Remove sliced string string type

As a first step to reduce the complexity of the string hierachy the sliced string type is removed. Whenever a sub-string is created it is allocated as a fresh flat string.
Review URL: http://codereview.chromium.org/385004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3259 ce2b1a6d-e550-0410-aec6-3dcde31c8c00