Rust symbol demangling is not supported

There is no way to see the names of Rust symbols. The same name `[unknown]` appears every time for every function in the stack trace. For example, the stack after running `offcputime` looks like this:
```
try-linux-futex;[unknown];[unknown];[unknown];[unknown];[unknown];[unknown];[unknown];[unknown];[unknown];-;entry_SYSCALL_64_after_hwframe;do_syscall_64;__x64_sys_clock_nanosleep;common_nsleep;hrtimer_nanosleep;do_nanosleep;schedule;__schedule;finish_task_switch.isra.0 5000186
```

BTW, in `bpftrace` project there is the [same](https://github.com/bpftrace/bpftrace/issues/3532) problem.
As for `perf`, it handles Rust symbols' names correctly.