Refactor and speed up mj_crb and add a benchmark.

This is a no-op "prefactor" of `mj_crb` to reduce the number of lines changed in the upcoming sleeping CL. The main change here is that `mj_crb` now avoids accessing the model and data pointer repeatedly, but instead has the input and output pointers explicitly declared as local variables. A benchmark test found a healthy **14% perf bump** due to two changes:
- Adding `restrict` to `mju_mulInertVec`
- The function-local pointers.

Adding `restrict` to the local pointers had no effect. Note that `mj_crb` is not a particularly expensive function so these speed bumps are not significant per se, but rather indicative of possible future gains with these techniques.

```
Benchmark                Time(ns)        CPU(ns)     Iterations
---------------------------------------------------------------

ORGINAL BASELINE
BM_CRB_BASELINE_mean         4325           4348         993200  230.052k items/s

BASELINE + RESTRICT
BM_CRB_BASELINE_mean         4057           4090        1186150  244.573k items/s

LOCAL POINTERS + RESTRICT
BM_CRB_mean                  3772           3800        1001750  263.209k items/s
```

PiperOrigin-RevId: 815798225
Change-Id: Iffdf57b544e8e10562807c617f73ca4ccd1c614f

test/benchmark/CMakeLists.txt

@@ -67,6 +67,12 @@ mujoco_test(
   ADDITIONAL_LINK_LIBRARIES benchmark::benchmark absl::core_headers
 )
 
+mujoco_test(
+  crb_benchmark_test
+  MAIN_TARGET benchmark::benchmark_main
+  ADDITIONAL_LINK_LIBRARIES benchmark::benchmark absl::core_headers
+)
+
 mujoco_test(
   engine_util_sparse_benchmark_test
   MAIN_TARGET benchmark::benchmark_main