Enable the rust_2018_idioms lint as warning

[hhirtz]

This should become the default in the future:
rust-lang/rust#54910

Reason this is put in lib.rs instead of CI: to ensure local invocations
of cargo build and cargo clippy pick up the warning.

Reason this is made a warning: so that code builds and runs anyway. On
the CI, the -D warnings clippy argument will treat it as a hard error.

[codecov]

Codecov Report

Merging #171 (bd9f9b1) into master (b6b1c43) will increase coverage by 0.02%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
+ Coverage   50.66%   50.69%   +0.02%     
==========================================
  Files          41       41              
  Lines        7016     7019       +3     
==========================================
+ Hits         3555     3558       +3     
  Misses       3461     3461              

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)
src/topology.rs	37.31% <33.33%> (ø)
src/algorithms/fiduccia_mattheyses.rs	89.88% <100.00%> (ø)
src/algorithms/graph_growth.rs	98.07% <100.00%> (ø)
src/algorithms/k_means.rs	80.25% <100.00%> (ø)
src/algorithms/kernighan_lin.rs	93.83% <100.00%> (ø)
src/algorithms/recursive_bisection.rs	92.48% <100.00%> (ø)
tools/src/bin/part-bench/main.rs	0.52% <0.00%> (-0.01%)	⬇️
src/algorithms/vn/first.rs	73.30% <0.00%> (+1.19%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b6b1c43...bd9f9b1. Read the comment docs.