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/Codec/GuidStringCodecTest.php

@@ -56,6 +56,7 @@ class GuidStringCodecTest extends TestCase
             ->method('getFields')
             ->willReturn($this->fields);
         $codec = new GuidStringCodec($this->builder);
+        /** @phpstan-ignore method.resultUnused */
         $codec->encode($this->uuid);
     }