added dupe method

tests/root.zig

@@ -36,3 +36,35 @@ test "heap" {
     try std.testing.expectEqual(s.len, str.len());
     try std.testing.expectEqualStrings(s, str.str());
 }
+
+test "dupe inline" {
+    const alloc = std.testing.allocator;
+
+    {
+        const s = "abcd";
+
+        const str1 = try SmolStr.init(alloc, s);
+        defer str1.deinit(alloc);
+
+        const str2 = try str1.dupe(alloc);
+        defer str2.deinit(alloc);
+
+        try std.testing.expectEqualSlices(u8, std.mem.asBytes(&str1), std.mem.asBytes(&str2));
+    }
+
+    {
+        const s = "abcdefghijklmnopqrstuvwxyz";
+
+        const str1 = try SmolStr.init(alloc, s);
+        defer str1.deinit(alloc);
+
+        const str2 = try str1.dupe(alloc);
+        defer str2.deinit(alloc);
+
+        // try std.testing.expectEqualSlices(u8, std.mem.asBytes(&str1), std.mem.asBytes(&str2));
+
+        try std.testing.expectEqualStrings(str1.str(), str2.str());
+
+        try std.testing.expect(str1.inner.heap.ptr != str2.inner.heap.ptr);
+    }
+}