src: simplify node::Utf8Value()

* Remove kStorageSize constant.

* Remove superfluous local variable and reinterpret_cast.

* Reorder data members so the length field and data pointer
  (if not the data itself) fit in a single cache line.

PR-URL: https://github.com/iojs/io.js/pull/1042


Reviewed-By: Trevor Norris <trev.norris@gmail.com>