string compress

[jianshu93]

Hello needle tail, team,

is that possible to compress AGCT character, because there are at most 5 possibilities (PLUS N) of the string, we need 3 bit to encode the 5 possibilities instead of 1 byte for a character.

Thanks,

Jianshu

[Keats]

https://docs.rs/needletail/0.4.1/needletail/bitkmer/index.html does it but only for ATCG. It's not super hard to handle additional characters but it's unlikely to be added to this crate.