Make dbg_* methods return the inspected variable
This is intended as a small QoL improvement for devs: The idea is that if you want to investigate the branching in
```cpp
if (long_complicated_condition)
```
you can just use
```cpp
if (dbg_hit_on(long_complicated_condition))
```
instead of having to do
```cpp
dbg_hit_on(long_complicated_condition);
if (long_complicated_condition)
```
or something even more complicated if evaluating the condition has side effects.
Same idea for `dbg_mean_of`, `dbg_stdev_of` and `dbg_extremes_of`,
closes https://github.com/official-stockfish/Stockfish/pull/7121
No functional change