Replace low-level tree positioning code with tsk_tree_position_t calls

[jeromekelleher]

Moving to the next and previous tree in a sequence is currently implemented in tsk_tree_advance and seeking to a position along the sequence (from the initial state) is done in tsk_tree_seek_from_null .

We can replace this logic with calls to tsk_tree_position_t, following the patterns used in #2786.

We should do some basic performance testing to check that we haven't introduced any regressions by doing this, say by checking on

• A large msprime simulated tree sequence
• A large chromosome from the unified genealogy trees
• A SARS-Cov-2 tree sequence

Note that we didn't implement seek_backwards in #2786, as I thought this would be a good exercise for someone learning how all this stuff works. The first part of closing this issue would be a commit that implements seek_backwards in Python, and adds some more testing to ensure it's correct.

Note that this doesn't address the issues in #2792, but that can be done as a follow-up, since we're not going to make performance any worse by doing it this way.

We would continue to implement seek by iteration for the non-null case, because we're less sure about the performance implications of this, and we'd need some additional testing to verify that it's correct.

@duncanMR - are you up for this?

[duncanMR]

@jeromekelleher Yes, I'd be happy to implement seek_backwards and do some performance tests. Just to confirm: are the unit tests for seek_forward mostly complete in #2786, such that I just need to translate them to the seek_backward case?

[jeromekelleher]

Great!

Testing is almost complete I think, but we probably need a few more tests for seeking randomly along the sequence, and changing directions.