populate remaining missing return types, using native returns where possible

this is done separately to stable so that stable changes can be integrated easily using an empty merge.

src/utils/Config.php

@@ -319,7 +319,7 @@ class Config{
 	 * @param string $k
 	 * @param mixed  $v
 	 */
-	public function __set($k, $v){
+	public function __set($k, $v) : void{
 		$this->set($k, $v);
 	}
 

src/utils/MainLogger.php

@@ -151,6 +151,8 @@ class MainLogger extends \AttachableThreadedLogger{
 	/**
 	 * @param \Throwable $e
 	 * @param array|null $trace
+	 *
+	 * @return void
 	 */
 	public function logException(\Throwable $e, $trace = null){
 		if($trace === null){