#3317 Added missingness support for alignments via a pure-Python implementation. Implement a C API for this and use it. Something like
tsk_tree_sequence_alignments(tsk_tree_sequence_t *self, const char *ref_seq, tsk_size_t ref_seq_length, tsk_id_t *nodes, double* left, double *right, char **alignments)? Although we don't have any existing C API that returns what could be a huge chunk of memory like this. We could consider an iterator like the variants API, decoding a sensible amount of nodes at a time for efficiency.