should bam::Record::push_aux be infallible?

[pmarks]

The only way that push_aux can fail is on OOM, or if an invalid type code is passed. The Aux enum should prevent an invalid type code from being passed. There are a bunch of other functions (eg set_qname and related) that can fail due to OOM, but don't propagate an error. I find myself adding a lot of error handling to non-IO-containing code, just to propagate errors from push_aux. I think we should remove the Result<> return, even though it would be a breaking change.

push_aux impl:
https://github.com/samtools/htslib/blob/673813297cee2c3235a41f9406ad2aa9494de954/sam.c#L2246

Note: in looking at the set_qname code path, I discovered that there's a memory safety bug here:
https://docs.rs/rust-htslib/0.24.0/src/rust_htslib/bam/record.rs.html#395
If realloc fails it will return NULL. We should check that condition and panic, rather than proceeding. I'll make a PR for that.

[johanneskoester]

Good catch. I agree, feel free to make that breaking change.