libutil: add should emplace inside a ChunkedVector

This simplifies many call-sites where construction can take place
automatically.

Change-Id: I87f697d55375676345b388024eb8df900bf808de
Co-authored-by: Tom Hubrecht <github@mail.hubrecht.ovh>
Signed-off-by: Raito Bezarius <raito@lix.systems>

lix/libexpr/symbol-table.hh

@@ -90,7 +90,7 @@ public:
         auto it = symbols.find(s);
         if (it != symbols.end()) return Symbol(it->second.second + 1);
 
-        const auto & [rawSym, idx] = store.add(std::string(s));
+        const auto & [rawSym, idx] = store.add(s);
         symbols.emplace(rawSym, std::make_pair(&rawSym, idx));
         return Symbol(idx + 1);
     }

lix/libutil/chunked-vector.hh

@@ -45,7 +45,8 @@ public:
 
     uint32_t size() const { return size_; }
 
-    std::pair<T &, uint32_t> add(T value)
+    template<typename... Args>
+    std::pair<T &, uint32_t> add(Args &&... args)
     {
         const auto idx = size_++;
         auto & chunk = [&] () -> auto & {
@@ -53,7 +54,7 @@ public:
                 return back;
             return addChunk();
         }();
-        auto & result = chunk.emplace_back(std::move(value));
+        auto & result = chunk.emplace_back(std::forward<Args>(args)...);
         return {result, idx};
     }