Rename str2var to str_to_var and similar

Affects the Math class, a good chunk of the audio code, and a lot of other miscellaneous classes, too.

- `var2str` -> `var_to_str`
- `str2var` -> `str_to_var`
- `bytes2var` -> `bytes_to_var`
- `bytes2var_with_objects` -> `bytes_to_var_with_objects`
- `var2bytes` -> `var_to_bytes`
- `var2bytes_with_objects` -> `var_to_bytes_with_objects`
- `linear2db` -> `linear_to_db`
- `db2linear` -> `db_to_linear`
- `deg2rad` -> `deg_to_rad`
- `rad2deg` -> `rad_to_deg`

- `dict2inst` -> `dict_to_inst`
- `inst2dict` -> `inst_to_dict`

scene/gui/video_stream_player.cpp

@@ -362,7 +362,7 @@ void VideoStreamPlayer::set_volume_db(float p_db) {
 	if (p_db < -79) {
 		set_volume(0);
 	} else {
-		set_volume(Math::db2linear(p_db));
+		set_volume(Math::db_to_linear(p_db));
 	}
 }
 
@@ -370,7 +370,7 @@ float VideoStreamPlayer::get_volume_db() const {
 	if (volume == 0) {
 		return -80;
 	} else {
-		return Math::linear2db(volume);
+		return Math::linear_to_db(volume);
 	}
 }