[pzstd] Fixes for Windows build

* Add `Portability.h` to fix min/max issues.
* Fix conversion warnings
* Assert that windowLog <= 23, which is currently always the case.
  This could be loosened, but we aren't looking to add new functionality.

Fixes on top of PR #3375 by @eli-schwartz, which added Windows CI for contrib & programs.

contrib/pzstd/utils/Portability.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2016-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ */
+
+#pragma once
+
+#include <algorithm>
+
+// Required for windows, which defines min/max, but we want the std:: version.
+#undef min
+#undef max