Replace string formatting and appending with write!

[bmesuere]

This pull request replaces string formatting and appending with write!

Benchmark on my laptop:

Benchmark: Short reads (NC_000913-454.fna)
Benchmark 1: /Users/bart/Code/FragGeneScanRs/target/release/FragGeneScanRs -s /Users/bart/Code/FragGeneScanRs/example/NC_000913-454.fna -t 454_10 -w 0 -o /var/folders/j3/38fskpy159v07np8syk3p_2m0000gn/T/tmp.9HS23tgInp/NC_000913-454
  Time (mean ± σ):     440.7 ms ±   6.7 ms    [User: 428.2 ms, System: 9.1 ms]
  Range (min … max):   432.6 ms … 460.3 ms    20 runs


Benchmark: Complete genome (NC_000913.fna)
Benchmark 1: /Users/bart/Code/FragGeneScanRs/target/release/FragGeneScanRs -s /Users/bart/Code/FragGeneScanRs/example/NC_000913.fna -t complete -w 1 -o /var/folders/j3/38fskpy159v07np8syk3p_2m0000gn/T/tmp.9HS23tgInp/NC_000913
  Time (mean ± σ):     642.2 ms ±  24.9 ms    [User: 513.9 ms, System: 123.9 ms]
  Range (min … max):   622.2 ms … 718.4 ms    20 runs


Benchmark: Long reads (contigs.fna)
Benchmark 1: /Users/bart/Code/FragGeneScanRs/target/release/FragGeneScanRs -s /Users/bart/Code/FragGeneScanRs/example/contigs.fna -t complete -w 1 -o /var/folders/j3/38fskpy159v07np8syk3p_2m0000gn/T/tmp.9HS23tgInp/contigs
  Time (mean ± σ):      3.931 s ±  0.009 s    [User: 3.882 s, System: 0.040 s]
  Range (min … max):    3.921 s …  3.950 s    10 runs

short reads: 440.7 ms ± 6.7 ms
complete genome: 642.2 ms ± 24.9 ms
long reads: 3.931 s ± 0.009 s

See #17 for a comparison.