More precise type annotations

[sasezaki]

continuous of #14
close #11

This changes reduces PHPStan error at --level=6 except for deprecated __call() method.

phpstan analyse --level=6 src
 3/3 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

 ------ ----------------------------------------------------------------------------------------
  Line   Test/TestLogger.php
 ------ ----------------------------------------------------------------------------------------
  112    Method Psr\Log\Test\TestLogger::__call() has no return type specified.
         🪪  missingType.return
  112    Method Psr\Log\Test\TestLogger::__call() has parameter $args with no type specified.
         🪪  missingType.parameter
  112    Method Psr\Log\Test\TestLogger::__call() has parameter $method with no type specified.
         🪪  missingType.parameter
 ------ ----------------------------------------------------------------------------------------



 [ERROR] Found 3 errors