Migrate to type-safe model ID structs

PluralKit.Core/Models/SystemId.cs

@@ -0,0 +1,24 @@
+namespace PluralKit.Core
+{
+    public readonly struct SystemId: INumericId<SystemId, int>
+    {
+        public int Value { get; }
+
+        public SystemId(int value)
+        {
+            Value = value;
+        }
+
+        public bool Equals(SystemId other) => Value == other.Value;
+
+        public override bool Equals(object obj) => obj is SystemId other && Equals(other);
+
+        public override int GetHashCode() => Value;
+
+        public static bool operator ==(SystemId left, SystemId right) => left.Equals(right);
+
+        public static bool operator !=(SystemId left, SystemId right) => !left.Equals(right);
+
+        public int CompareTo(SystemId other) => Value.CompareTo(other.Value);
+    }
+}