Fix pure annotations (#605)

Coming from https://github.com/ramsey/uuid/pull/603, this is an attempt
to fix the errors raised by the current phpstan settings.

I went through each of the errors raised by phpstan with the following
approach.

- If a method is part of an `@immutable` class, we can consider it pure,
  assuming it only affects internal variables.
- If a potentially pure method is calling a class's method that is only
  swapped during testing (and not during normal usage), then we can
  consider the calling method pure.
- If a class is marked deprecated, don't bother with attempting to mark
  it pure or immutable.

tests/Encoder/TimestampFirstCombCodecTest.php

@@ -75,6 +75,7 @@ class TimestampFirstCombCodecTest extends TestCase
                     'node' => '0800200c9a66',
                 ]))
             );
+        /** @phpstan-ignore method.resultUnused */
         $this->codec->decode('0800200c-9a66-11e1-9b21-ff6f8cb0c57d');
     }
 

tests/Encoder/TimestampLastCombCodecTest.php

@@ -78,6 +78,7 @@ class TimestampLastCombCodecTest extends TestCase
                     'node' => 'ff6f8cb0c57d',
                 ]))
             );
+        /** @phpstan-ignore method.resultUnused */
         $this->codec->decode('0800200c-9a66-11e1-9b21-ff6f8cb0c57d');
     }