Add read-update-write benchmark

Update the README with a new benchmark group and performance results.
Include new data files and update the Rust benchmark implementation.
Regenerate UPB bindings and fix a data path in the C benchmark runner.

upb_test/hackers_bench.c

@@ -66,7 +66,7 @@ typedef struct {
 } BenchData;
 
 static bool load_bench_data(BenchData *out, const char *name) {
-    snprintf(out->path, sizeof(out->path), "../data/bench/%s.pb", name);
+    snprintf(out->path, sizeof(out->path), "data/bench/%s.pb", name);
     FILE *f = fopen(out->path, "rb");
     if (!f) {
         printf("[skip] %s not found — "