Hash file names to speed up some operations.

In particular, zip_file_add and zip_name_locate.
Add consistency check for duplicate file names,
now that it's cheap.

lib/CMakeLists.txt

diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 3bfc622..2a0962f 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -100,6 +100,7 @@
   zip_get_name.c
   zip_get_num_entries.c
   zip_get_num_files.c
+  zip_hash.c
   zip_io_util.c
   zip_memdup.c
   zip_name_locate.c