Distinguish between empty match and non-match

[lucic71]

After 4cb7e30, the matcher inside mapper.cpp broke for the following case:

template:     "T1 & std::vector<T1>::back()"
instantiated: "& std::vector<int> & std::vector<std::vector<int>>::back()"

Leading & comes from the call inside VisitUnaryOperator

The first T1 is matched to the empty string:

T1 & std::vector<   T1>::back()
   & std::vector<   int> & std::vector<std::vector<int>>::back()

Then, when the second T1 is matched (int> & std::vector<std::vector<int>), it goes to the else branch because it believes that the first T1 was never encountered:

cpp2rust/cpp2rust/converter/mapper.cpp

Lines 253 to 260 in 1ac73cc

	if (!repl.empty()) {
	size_t end_pos = 0;
	if (!matchLiteralAt(instantiated, si, repl, end_pos)) {
	return std::nullopt;
	}
	si = end_pos;
	} else {
	if (!nextLit.empty()) {

The fix is to replace the captured std::vector<std::string> with std::vector<std::optional<std::string>> so that empty matched string can be distinguished from non-matched stirng.