Using iterators with matrices about cpp11

[jimhester]

I added a simple forward iterator, enough to do ranged based for loops e.g. the row_sums implementation at

cpp11/cpp11test/src/matrix.cpp

Lines 70 to 86 in 9cba571

	[[cpp11::register]] cpp11::doubles row_sums(cpp11::doubles_matrix x) {
	cpp11::writable::doubles sums(x.nrow());
	int i = 0;
	for (auto& row : x) {
	for (auto&& val : row) {
	if (cpp11::is_na(val)) {
	sums[i] = NA_REAL;
	break;
	}
	sums[i] += val;
	}
	++i;
	}
	return sums;
	}

We can add additional operators in the future