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/Type/Time.php

@@ -43,11 +43,17 @@ final class Time implements TypeInterface
         $this->microseconds = new IntegerObject($microseconds);
     }
 
+    /**
+     * @pure
+     */
     public function getSeconds(): IntegerObject
     {
         return $this->seconds;
     }
 
+    /**
+     * @pure
+     */
     public function getMicroseconds(): IntegerObject
     {
         return $this->microseconds;