Turn PgAttribute::Key into class instead of alias for a std::tuple.

This improves readability as the fields are now named instead of numbered.

pglablib/catalog/PgAttribute.h

@@ -21,7 +21,19 @@ public:
         Stored
     };
 
-	using Key = std::tuple<Oid, int16_t>;
+    class Key {
+    public:
+        Key() = default;
+        Key(Oid relationId, int16_t num)
+            : RelationId(relationId)
+            , Num(num)
+        {}
+
+        std::strong_ordering operator <=> (const Key &rhs) const = default;
+    private:
+        Oid RelationId = InvalidOid;
+        int16_t Num = 0;
+    };
 
 	Oid relid = InvalidOid;
 	QString name;
@@ -47,9 +59,22 @@ public:
 	QString sername, serschema; // serial sequence name and schema
     QString description; ///< from pg_description
 
-	bool operator==(Key _k) const { return relid == std::get<0>(_k) && num == std::get<1>(_k); }
+    Key key() const
+    {
+        return Key(relid, num);
+    }
+
+    std::strong_ordering operator <=> (const Key& rhs) const
+    {
+        return key() <=> rhs;
+    }
+
+    bool operator==(const Key &k) const
+    {
+        return key() == k;
+    }
+
 	bool operator==(const QString &n) const { return name == n; }
-	bool operator<(Key _k) const { return relid < std::get<0>(_k) || (relid == std::get<0>(_k) && num < std::get<1>(_k)); }
 	bool operator<(const PgAttribute &rhs) const { return relid < rhs.relid || (relid == rhs.relid && num < rhs.num); }
 
 	/// Return the part of the SQL create statement that can be reused for both the CREATE TABLE and ALTER TABLE ADD COLUMN