libexpr: Use table.size() instead of unnecessary loop

(cherry picked from commit d8fc55a)

Author: xokdvium

src/libexpr/primops/fromTOML.cc

@@ -17,14 +17,7 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value ** args, Va
         switch (t.type()) {
         case toml::value_t::table: {
             auto table = toml::get<toml::table>(t);
-
-            size_t size = 0;
-            for (auto & i : table) {
-                (void) i;
-                size++;
-            }
-
-            auto attrs = state.buildBindings(size);
+            auto attrs = state.buildBindings(table.size());
 
             for (auto & elem : table) {
                 forceNoNullByte(elem.first);