Add fresh concatStringsSep without bug

The buggy version was previously renamed to
dropEmptyInitThenConcatStringsSep

src/libutil/strings-inline.hh

@@ -0,0 +1,31 @@
+#pragma once
+
+#include "strings.hh"
+
+namespace nix {
+
+template<class C>
+std::string concatStringsSep(const std::string_view sep, const C & ss)
+{
+    size_t size = 0;
+    bool tail = false;
+    // need a cast to string_view since this is also called with Symbols
+    for (const auto & s : ss) {
+        if (tail)
+            size += sep.size();
+        size += std::string_view(s).size();
+        tail = true;
+    }
+    std::string s;
+    s.reserve(size);
+    tail = false;
+    for (auto & i : ss) {
+        if (tail)
+            s += sep;
+        s += i;
+        tail = true;
+    }
+    return s;
+}
+
+} // namespace nix