Optimize float formatting

Kernel.sprintf is faster then String#%

    require 'benchmark/ips'

    FLOAT = 0.025

    def percent(x)
      "%.2f" % x
    end

    def kernel(x)
      Kernel.sprintf "%.2f", x
    end

    Benchmark.ips do |x|
      x.report "percent" do
        percent FLOAT
      end

      x.report "kernel" do
        kernel FLOAT
      end
    end

    # Calculating -------------------------------------
    #              percent     41064 i/100ms
    #               kernel     42965 i/100ms
    # -------------------------------------------------
    #              percent   704509.3 (±1.3%) i/s -    3531504 in
    #              5.013556s
    #               kernel   786417.1 (±1.8%) i/s -    3952780 in
    #               5.028099s
    #