Get rid of the use of str.repeat()

[lilyball]

This allocates a string. Instead we can (ab)use format specifiers to do the repetitions for us.

This produces a very mild speedup:

Benchmark #1: ./original_hexyl original_hexyl
  Time (mean ± σ):      1.482 s ±  0.016 s    [User: 1.426 s, System: 0.050 s]
  Range (min … max):    1.468 s …  1.516 s
 
Benchmark #2: ./target/release/hexyl original_hexyl
  Time (mean ± σ):      1.473 s ±  0.002 s    [User: 1.423 s, System: 0.046 s]
  Range (min … max):    1.472 s …  1.480 s
 
Summary
  './target/release/hexyl original_hexyl' ran
    1.01 ± 0.01 times faster than './original_hexyl original_hexyl'

[sharkdp]

That's pretty cool! Thank you very much.