✨ scrape descriptions on encode

[dmiller15]

Followup work to #34.

I tried to tackle the task of INFO field description scraping requested in #33. Found rust-htslib pretty ill-equipped for the task, but I'm way too much of a rust/c novice to delve deeply into the core of htslib. This could perhaps be done better using bcf_hdr_get_hrec.

In the end I made a short function to filter down the HeaderRecord vector using an ID value. Since there should only ever be one INFO header record with a particular ID, I return the description from the first of the remaining records. If it's not there I pass along the default value. On the way out I make sure to trim the " since the update_header function expects the descriptions to be unquoted.

In the encoder_main function I only call this function when the field description is not the default. I figure we'd want to give user priority in field naming.

As for how the check against the default is done, I couldn't really figure out how to get a default value for a field the way things were so I went ahead and made the fields::default_description_string() function public. I'm not sure if that's a good way of doing that so I'm open to suggestions.

Let me know if this is fine. Feel free to discard the PR if you want to go about this a different way.

[brentp]

@dmiller15 , this looks good to me. Ready from your end?

[dmiller15]

Just tested it, and it's still working as intended.

[brentp]

thank you!

[brentp]

@dmiller15 , this fails tests/big.sh. Can you fix that?

[dmiller15]

Sure I'll take a look

[dmiller15]

Ok didn't notice that we'd added more tests, and the test logic is no longer true. I'll update that batch of tests and have a PR up today.