is_valid_optimized_type() rejects the markers BJData excludes, so the reader
no longer needs its own list; the test now checks is_valid_optimized_type()
of a BJData reader against the writer's list. The BSON size comment now says
that open_bson_document() rejects a size below 5.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>