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/Math/BrickMathCalculatorTest.php

@@ -98,6 +98,7 @@ class BrickMathCalculatorTest extends TestCase
         $this->expectException(InvalidArgumentException::class);
         $this->expectExceptionMessage('"o" is not a valid character in base 16');
 
+        /** @phpstan-ignore method.resultUnused */
         $calculator->fromBase('foobar', 16);
     }