Function backend_of 

pub fn backend_of(codec: &str) -> &'static str

Which implementation of codec this build benches: "c" (a C library) or "pure" (Rust). Only zstd, xz and lzma have a C tier among the registry’s codecs. zlib-c and lz4-c do not count: they serve the gzip and lz4 compatibility links, while the gzip, zlib, deflate and lz4 codecs bench runs stay on flate2’s miniz_oxide and on lz4_flex in every build. lzma’s C tier is stuffr’s lzma-c, which this crate turns on only through c-backed (and full, which names it).