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/benchmark/UuidStringConversionBench.php

@@ -208,16 +208,19 @@ final class UuidStringConversionBench
 
     public function benchStringConversionOfTinyUuid(): void
     {
+        /** @phpstan-ignore method.resultUnused */
         $this->tinyUuid->toString();
     }
 
     public function benchStringConversionOfHugeUuid(): void
     {
+        /** @phpstan-ignore method.resultUnused */
         $this->hugeUuid->toString();
     }
 
     public function benchStringConversionOfUuid(): void
     {
+        /** @phpstan-ignore method.resultUnused */
         $this->uuid->toString();
     }
 
@@ -230,16 +233,19 @@ final class UuidStringConversionBench
 
     public function benchBytesConversionOfTinyUuid(): void
     {
+        /** @phpstan-ignore method.resultUnused */
         $this->tinyUuid->getBytes();
     }
 
     public function benchBytesConversionOfHugeUuid(): void
     {
+        /** @phpstan-ignore method.resultUnused */
         $this->hugeUuid->getBytes();
     }
 
     public function benchBytesConversionOfUuid(): void
     {
+        /** @phpstan-ignore method.resultUnused */
         $this->uuid->getBytes();
     }