With use_size, a container is announced by the marker `#` and its element
count as an integer, three small sink writes for the many containers of up to
127 elements (an empty array, an object with a few members).
A count of up to 127 is now written as `#`, `i` and one byte in a single
reservation.
Instructions per call (Linux, GCC 13, thousands), to_ubjson with use_size and use_type: canada 25813 -> 22060, citm_catalog 10656 -> 9192, 20000 small objects 18178 -> 16798; to_bjdata with use_size and use_type: canada 26341 -> 22588, citm_catalog 10904 -> 9440.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
26b4716740