Make the index text of items() on arrays lazy

basic_json_view::items() on an array formatted every element's index with
std::to_string when the item was made, whether or not the loop asks for the
key. The item now keeps the position and makes the text on the first key()
call. Objects are not affected.

Iterating over the items of an array of 3000 numbers and reading only the values:
  Apple M1: 25.5 us -> 4.6 us (0.45 M -> ~0 net instructions)
  Linux x86-64 Ir (whole run), GCC 13 / Clang 18: 14.9 M -> 4.1 M / 15.3 M -> 4.6 M
Reading key() costs what it did (24.5 us).

Belongs to the PR "json_view: read-side speedups".

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view.cpp

@@ -1113,6 +1113,28 @@ TEST_CASE("json_view element access and iteration")
         CHECK((*first++).key() == "0");
         CHECK((*first).key() == "1");
         CHECK(++first == items.end());
+
+        // the keys of array elements are their indexes, whether or not they are asked for
+        std::string numbers = "[0";
+        for (int i = 1; i < 1234; ++i)
+        {
+            numbers += "," + std::to_string(i);
+        }
+        const json_document array = json_document::parse(numbers + "]");
+        std::size_t position = 0;
+        for (const auto& item : array.root().items())
+        {
+            CHECK(item.value().get<std::size_t>() == position);
+            if (position % 3 != 0) // (not for every item: values alone do not make the key)
+            {
+                CHECK(item.key() == std::to_string(position));
+                CHECK(item.key().data() == item.key().data()); // (the same text every time)
+                const json_view::item copy = item;
+                CHECK(copy.key() == std::to_string(position));
+            }
+            ++position;
+        }
+        CHECK(position == 1234);
 #ifdef JSON_HAS_CPP_17
         std::string pairs;
         for (const auto [key, value] : d.root().items())