message: refactor: make function get_comp_level() infallible
  This function can never fail. Here I remove a call to unwrap(), which can never occur, by replacing traits, and remove error return type
diff: @@ -7,7 +7,7 @@ use flate2::write::GzEncoder;
  use flate2::Compression as GzCompressionLevel;
  use log::debug;
  use num::Integer;
  -use num_traits::NumCast;
  +use num_traits::{FromPrimitive, NumCast, ToPrimitive};
  use std::env;
  use std::io::{ErrorKind, Read, Write};
  use std::path::Path;
  @@ -133,12 +133,12 @@ impl<'r> Read for Decompressor<'r> {
  }
  }
  -fn get_comp_level<I: FromStr + Integer + NumCast>(ext: &str) -> I {
  +fn get_comp_level<I: FromStr + From<u8>>(ext: &str) -> I {
  let var_name = format!("{}_COMPRESSION", ext.to_uppercase());
  env::var(var_name)
  .ok()
  .and_then(|val| val.parse::<I>().ok())
  - .unwrap_or_else(|| NumCast::from(2).unwrap())
  + .unwrap_or_else(|| I::from(2))
  }
  pub struct Compressor<'w> {
  