Seek/Tell for BGZF Module

[rmhubley]

The bgzf reader/writer implemented here is super fast, however it appears (perhaps I am missing something in the docs) to not expose the full bgzf.c functionality provided by htslib. Specifically the bgzf_useek() and bgzf_utell() functionality. These are essential for using bgzf files in-place. I realize that the Noodles reimplementation of the bgzf module does support them, however currently the Rust port is a less efficient reader/writer than the C implementation.