Perf. improvements to Hash#to_json in pure implementation generator.

[vipulnsward]

Perf. improvements to Hash#to_json in pure implementation generator:

• Use .each instead of for for iterating over elements.
• Initialize delim, result vars with initial states instead of appending state.object_nl later.
• Use faster string interpolation where possible on Hash#json_transform instead of appending strings repeatedly.

Benchmark:

    hash ={:a => {:a => {:a => {:a => {:a => 1}}}}}
    require 'benchmark/ips'
    Benchmark.ips do |x|
      x.report("to_json ") {  hash.to_json}
    end

Before:
Calculating -------------------------------------
            to_json       4990 i/100ms
-------------------------------------------------
            to_json     52970.8 (±5.9%) i/s -     264470 in   5.010558s

After:
Calculating -------------------------------------
            to_json       5253 i/100ms
-------------------------------------------------
            to_json     55998.6 (±5.9%) i/s -     283662 in   5.083350s

[marcandre]

This PR has conflicts.
There are some gains here, but they are pretty modest and I think the actually best way to improve the performance would be to build and array and join it at the very end (although I have not tried nor benchmarked this).
I would close this PR.

[hsbt]

@vipulnsward Sorry, too late response. I update with the current HEAD and merged!