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

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