Refactor converting number to string by using optimized function

[Watson1978]

Refactor converting number to string by using optimized function which introduced at #879

[ohler55]

Before merging, is the performance the same or better with this change?

[Watson1978]

Sorry for late reply.

When I run benchmark with same code in #879,
it is 0.8% slower by this PR.

I believe it is a tolerance error.

Before

Warming up --------------------------------------
             Oj.dump   431.211k i/100ms
Calculating -------------------------------------
             Oj.dump      4.323M (± 0.7%) i/s -     21.992M in   5.087412s

After

Warming up --------------------------------------
             Oj.dump   425.450k i/100ms
Calculating -------------------------------------
             Oj.dump      4.288M (± 0.3%) i/s -     21.698M in   5.060483s