Simplify the loop for parsing Integer string

[Watson1978]

There are several branches in the loop that parse Integer string.
In order to speed up parsing, this patch will move unwanted branches out of the loop.

−	before	after	result
integer	4.860M	5.088M	1.045x
float	4.845M	5.084M	1.049x

Environment

• Linux
  • Manjaro Linux x86_64
  • Kernel: 5.18.12-3-MANJARO
  • AMD Ryzen 7 5700G
  • gcc version 12.1.0
  • Ruby 3.1.2

Before

Warming up --------------------------------------
     Oj.load integer   488.037k i/100ms
       Oj.load float   485.863k i/100ms
Calculating -------------------------------------
     Oj.load integer      4.860M (± 0.1%) i/s -     48.804M in  10.042684s
       Oj.load float      4.845M (± 0.3%) i/s -     48.586M in  10.028592s

After

Warming up --------------------------------------
     Oj.load integer   508.044k i/100ms
       Oj.load float   508.281k i/100ms
Calculating -------------------------------------
     Oj.load integer      5.088M (± 0.2%) i/s -     51.312M in  10.085310s
       Oj.load float      5.084M (± 0.2%) i/s -     51.336M in  10.098332s

Test code

require 'benchmark/ips'
require 'oj'

Benchmark.ips do |x|
  x.warmup = 3
  x.time = 10

  json_integer = Oj.dump(2 ** 30)
  x.report('Oj.load integer') do |times|
    i = 0
    while i < times
      Oj.load(json_integer)
      i += 1
    end
  end

  json_float = Oj.dump(1234.123456789)
  x.report('Oj.load float') do |times|
    i = 0
    while i < times
      Oj.load(json_integer)
      i += 1
    end
  end
end