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.

src/Fields/FieldsInterface.php

@@ -26,6 +26,8 @@ interface FieldsInterface extends Serializable
 {
     /**
      * Returns the bytes that comprise the fields
+     *
+     * @pure
      */
     public function getBytes(): string;
 }