Add filesystem index to ZipOS

This change brings the /zip/... read-only filesystem into performance
parity with the native Linux filesystem which doesn't use compression
therefore, imagine how much faster this could be with bloom filtering
rather than simple binary search, and if we used zstd instead of zlib

test/libc/stdio/zipdir_test.c

@@ -87,6 +87,7 @@ TEST(__zipos_normpath, overflows_willNulTerminate) {
 TEST(__zipos_normpath, vectors) {
   static const char V[][2][128] = {
       {"", ""},
+      {"/", ""},
       {"/..", ""},
       {"/../", ""},
       {".", ""},