add top_procs_running.erl

Shows which kinds of processes are running in the system (grouping by initial_call MFA).
Useful for analyzing process leaks.

Example:

```erlang
1> TopProcsRunning().
[{{proc_lib,init_p,5},12},
 {{erlang,apply,2},8},
 {{group,server,3},2},
 {{application_master,start_it,4},1},
 {{user_drv,server,2},1},
 {{otp_ring0,start,2},1}]
```

Example 2:

```erlang
2> ProcsGroupBy(current_function).
[{{gen_server,loop,6},11},
 {{group,server_loop,3},2},
 {{application_master,loop_it,4},1},
 {{user_drv,server_loop,5},1},
 {{standard_error,server_loop,1},1},
 {{code_server,loop,1},1},
 {{global,loop_the_locker,1},1},
 {{gen_event,fetch_msg,5},1},
 {{erl_prim_loader,loop,3},1},
 {{erl_eval,do_apply,6},1},
 {{init,loop,1},1},
 {{global,loop_the_registrar,0},1},
 {{shell,shell_rep,4},1},
 {{application_master,main_loop,2},1}]
```