eliminate find of "find + emplace/insert" sequence

[rockeet]

auto iter = map.find(key);
if (iter == map.end()) { // not found
  map.emplace(...); // or insert
} else {
  // found, handle dup
}

can be optimized to:

auto [iter, success] = map.emplace(key_value); // or insert
if (!success) {
  // found, handle dup
}

The optimized code eliminated the redundant calling to find.