More simplification (with a bonus performance boost)

[tertsdiepraam]

• Extract snippets like this into a div_ceil function:

let mut num_lines = self.cells.len() / num_columns;
if self.cells.len() % num_columns != 0 {
    num_lines += 1;
}

• Simplify the Display impl, by precalculating the separator and using the padding functionality of format!.
• Simplify theoretical_max_num_lines by using enumerate and only deal with the widths instead of the the whole Cell.
• Remove a clone in width_dimensions.
• Add an example that takes a few seconds for benchmarking.

Once I had the example, I benchmarked this against main and already got a >2.58x improvement!

Benchmark 1: target/bench/old
  Time (mean ± σ):      4.412 s ±  0.048 s    [User: 3.738 s, System: 0.670 s]
  Range (min … max):    4.354 s …  4.507 s    10 runs
 
Benchmark 2: target/bench/new
  Time (mean ± σ):      1.709 s ±  0.014 s    [User: 1.085 s, System: 0.624 s]
  Range (min … max):    1.694 s …  1.739 s    10 runs
 
Summary
  'target/bench/new' ran
    2.58 ± 0.03 times faster than 'target/bench/old'

EDIT: Got another performance boost from 2.1x to 2.5x faster

[tertsdiepraam]

Note entirely sure what's going on but there's some weird things happening in this branch when I try to run all the firefox files.

[codecov]

Codecov Report

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@@           Coverage Diff           @@
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  Coverage        ?   75.71%           
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  Files           ?        4           
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  Branches        ?       58           
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  Misses          ?       78           
  Partials        ?       16           

Flag	Coverage Δ
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ubuntu_latest	75.71% <0.00%> (?)
windows_latest	75.71% <0.00%> (?)

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[tertsdiepraam]

Alright, I'm putting this up for review anyway. The reason is that I want to have these cleanups before I start working on more fixes. It doesn't have to be perfect and we can fix more along the way.