rustdoc-search: compressed bitmap to sort, then load desc

This adds a bit more data than "pure sharding" by
including information about which items have no description
at all. This way, it can sort the results, then truncate,
then finally download the description.

With the "e" bitmap: 2380KiB

Without the "e" bitmap: 2364KiB

src/tools/rustdoc-js/tester.js

@@ -559,8 +559,3 @@ process.on("beforeExit", () => {
     console.log("process did not complete");
     process.exit(1);
 });
-
-/*process.on("uncaughtException", (err) => {
-    console.log(`Uncaught Exception: ${err.message}`);
-    process.exit(1);
-});*/