Adds two functions that take a range over pairs of serialized values and return a serialized map value. There are 2 functions - one operating on bytes and one operating on managed_bytes. The version with managed_bytes is used in expression.cc, used to be a local static function. The bytes version will be used in type_json.cc in the next commit. Signed-off-by: Jan Ciolek <jan.ciolek@scylladb.com>