Type cleanup, internally and in the API.  Fixes most llvm warnings.
diff --git a/lib/zip_dir_add.c b/lib/zip_dir_add.c
index a55b10c..0a74bd6 100644
--- a/lib/zip_dir_add.c
+++ b/lib/zip_dir_add.c
@@ -45,7 +45,7 @@
 ZIP_EXTERN zip_int64_t
 zip_dir_add(struct zip *za, const char *name, zip_flags_t flags)
 {
-    int len;
+    size_t len;
     zip_int64_t ret;
     char *s;
     struct zip_source *source;