Rename ZipEntryName to ZipString

since the struct is now used for other purposes. Also add some
comparator functions to the struct to simplify zip_archive.cc.

This is a follow-up CL for f1d3d3b2477a813805b71099c60e07d36dbb225a.

Bug: 21957428
Change-Id: I60d4171eeacc561d59226d946e9eb5f9c96d80cf
4 files changed