WIP: Copy combinations logic from demo

src/combotest.cpp

@@ -0,0 +1,22 @@
+#include <combo.hpp>
+
+#include <iostream>
+
+std::ostream &operator<<(std::ostream &o, const std::vector<int> &data) {
+    o << "[ ";
+    for (const auto &el: data) {
+        o << el << " ";
+    }
+    o << "]";
+    return o;
+}
+
+int main() {
+    std::vector<int> data{1, 2, 3, 4, 5};
+
+    for (const auto &combo: combinations(data, 3)) {
+        std::cout << combo << std::endl;
+    }
+
+    return EXIT_SUCCESS;
+}