Generalize internalization of substrings.

Make a template version of SubStringKey, which allows internalization of substrings of sequential and external strings.

R=dcarney@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/143223004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18910 ce2b1a6d-e550-0410-aec6-3dcde31c8c00