common: Extract UUID to its own class

Since the Mii database uses UUIDs very similar to the Accounts database, it makes no sense to not share code between them.

src/common/uuid.h

@@ -0,0 +1,45 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "common/common_types.h"
+
+namespace Common {
+
+constexpr u128 INVALID_UUID{{0, 0}};
+
+struct UUID {
+    // UUIDs which are 0 are considered invalid!
+    u128 uuid = INVALID_UUID;
+    constexpr UUID() = default;
+    constexpr explicit UUID(const u128& id) : uuid{id} {}
+    constexpr explicit UUID(const u64 lo, const u64 hi) : uuid{{lo, hi}} {}
+
+    explicit operator bool() const {
+        return uuid != INVALID_UUID;
+    }
+
+    bool operator==(const UUID& rhs) const {
+        return uuid == rhs.uuid;
+    }
+
+    bool operator!=(const UUID& rhs) const {
+        return !operator==(rhs);
+    }
+
+    // TODO(ogniK): Properly generate uuids based on RFC-4122
+    static UUID Generate();
+
+    // Set the UUID to {0,0} to be considered an invalid user
+    void Invalidate() {
+        uuid = INVALID_UUID;
+    }
+
+    std::string Format() const;
+    std::string FormatSwitch() const;
+};
+static_assert(sizeof(UUID) == 16, "UUID is an invalid size!");
+
+} // namespace Common