Add benchmark for format() vs round().to_s

[carlos-contreras]

This compares two alternatives for starting with a floating point
number, transform it into a formatted string with a specific number
of digits after the point.

[Arcovion]

Typo: format('%.2f' % NUM) → format('%.2f', NUM) or '%.2f' % NUM

[Arcovion]

Strangely, on my machine format is consistently faster than %:

require 'benchmark/ips'

NUM = 1.12678.freeze

def fast
  NUM.round(2).to_s
end

def avg
  format('%.2f', NUM)
end

def slow
  '%.2f' % NUM
end

Benchmark.ips do |x|
  x.report('Float#round') { fast }
  x.report('Kernel#format') { avg }
  x.report('String#%') { slow }
  x.compare!
end

Warming up --------------------------------------
         Float#round   128.526k i/100ms
       Kernel#format    96.841k i/100ms
            String#%    89.243k i/100ms
Calculating -------------------------------------
         Float#round      1.981M (± 0.3%) i/s -     10.025M in   5.061882s
       Kernel#format      1.505M (± 0.3%) i/s -      7.554M in   5.017527s
            String#%      1.241M (± 0.3%) i/s -      6.247M in   5.034723s

Comparison:
         Float#round:  1980506.7 i/s
       Kernel#format:  1505458.6 i/s - 1.32x  slower
            String#%:  1240793.9 i/s - 1.60x  slower

So might be worth including both in the benchmark

[carlos-contreras]

Thanks for the feedback @Arcovion, I updated the PR 👍