Use variables directly in `format!` macro

[JSAbrahams]

Summary

Gets rid of last nightly clippy errors.

[codecov[bot]]

Codecov Report

Merging #445 (f552f0e) into develop (f1b0646) will decrease coverage by 0.03%. The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           develop     #445      +/-   ##
===========================================
- Coverage    88.16%   88.13%   -0.03%     
===========================================
  Files          105      105              
  Lines        11391    11391              
===========================================
- Hits         10043    10040       -3     
- Misses        1348     1351       +3     

Impacted Files	Coverage Δ
src/io.rs	92.10% <0.00%> (ø)
src/parse/lex/tokenize.rs	97.58% <0.00%> (ø)
src/check/context/function/mod.rs	52.00% <25.00%> (ø)
src/main.rs	85.71% <33.33%> (ø)
src/check/context/clss/mod.rs	85.44% <71.42%> (-1.90%)	:arrow_down:
src/check/context/arg/mod.rs	96.00% <100.00%> (ø)
src/common/delimit.rs	76.92% <100.00%> (ø)