Fix all clippy and rustc warnings (beta toolchain version 0.1.75)

[primeos-work]

This fixes all new warnings except these two false positives (s. commit message):

$ cargo clippy --release
   Compiling butido v0.4.0
warning: unnecessary map of the identity function
  --> src/job/runnable.rs:74:59
   |
74 |                 .chain(git_author_env.as_ref().into_iter().map(|(k, v)| (k, v)))
   |                                                           ^^^^^^^^^^^^^^^^^^^^^ help: remove the call to `map`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_identity
   = note: `#[warn(clippy::map_identity)]` on by default

warning: unnecessary map of the identity function
  --> src/job/runnable.rs:75:59
   |
75 |                 .chain(git_commit_env.as_ref().into_iter().map(|(k, v)| (k, v)))
   |                                                           ^^^^^^^^^^^^^^^^^^^^^ help: remove the call to `map`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_identity

warning: `butido` (bin "butido") generated 2 warnings (run `cargo clippy --fix --bin "butido"` to apply 2 suggestions)
    Finished release [optimized] target(s) in 12.68s