Refactor: rewrite superimpose_styles function

[dandavison]

Slightly faster:

hyperfine --warmup 10 --min-runs 20 \                                                                                                                                             
                'target/release/delta --no-gitconfig < /tmp/delta-benchmark-input.gitdiff > /dev/null'
Benchmark 1: target/release/delta --no-gitconfig < /tmp/delta-benchmark-input.gitdiff > /dev/null                                                                                 
  Time (mean ± σ):     808.2 ms ±  20.0 ms    [User: 791.6 ms, System: 31.7 ms]                                                                                                   
  Range (min … max):   784.6 ms … 872.3 ms    20 runs                                                                                                                             

vs previous

hyperfine --warmup 10 --min-runs 20 \                                                    
                'target/release/delta --no-gitconfig < /tmp/delta-benchmark-input.gitdiff > /dev/null'
Benchmark 1: target/release/delta --no-gitconfig < /tmp/delta-benchmark-input.gitdiff > /dev/null                                        
  Time (mean ± σ):     830.0 ms ±   7.3 ms    [User: 818.1 ms, System: 31.4 ms]                                                                                                   
  Range (min … max):   818.0 ms … 844.8 ms    20 runs                                                                                                                             

~2.65% speedup