The `n_next` member of struct `bc_struct` is unused; the only code that
would use this member has been disabled since libbcmath has been
bundled nearly 17 years ago. Apparently, `n_next` has been designed to
build a linked list of free numbers, but it is doubtful whether that
would be an improvement over relying on the efficiency of the ZendMM,
so we remove the remaining support altogether.
The ABI break probably affects nobody, but would be okay for a new
minor version according to our release process, anyway.
Christoph M. Becker