Squashed commit of the following:

commit 26d181059989279a79c433cedcd893b4f52e42ee
Author: Francois Chagnon <francois.chagnon@jadedpixel.com>
Date:   Tue Sep 15 21:17:34 2015 +0000

    add config options for escape_slash

commit fa282334051b16df91ca097dd7304b46f3bc7719
Author: Francois Chagnon <francois.chagnon@jadedpixel.com>
Date:   Mon Feb 9 21:09:33 2015 +0000

    add forward slash to escape character

https://github.com/flori/json/commit/26c1769969

https://github.com/flori/json/commit/2f3f44c180

This constant isn't used, so lets remove it.

The global can go bad if the compactor runs, so we need to look up the
constant instead of caching it in a global.

https://github.com/flori/json/commit/0e99a9aac5

In where to convert Hash key to String for json, this patch will add shortcut for String/Symbol in Hash key.

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    65.000  i/100ms
Calculating -------------------------------------
                json    659.576  (± 1.5%) i/s -      3.315k in   5.027127s
```

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    78.000  i/100ms
Calculating -------------------------------------
                json    789.781  (± 2.7%) i/s -      3.978k in   5.041043s
```

```
require 'json'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id" => i,
    :age => 42,
  }
end

Benchmark.ips do |x|
  x.report "json" do |iter|
    count = 0
    while count < iter
      JSON.generate(obj)
      count += 1
    end
  end
end
```

https://github.com/flori/json/commit/38c0f6dbe4

To convert Hash convert, this part was using following pseudo code

```
obj.keys.each do |key|
  value = obj[key]
  ...
end
```

and `rb_funcall()` was called for `obj.keys`.
It might be slightly heavy to call the Ruby method.
This patch will iterate to convert Hash object about key/value using `rb_hash_foreach()` Ruby API instead of `rb_funcall()`.

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    55.000  i/100ms
Calculating -------------------------------------
                json    558.501  (± 1.1%) i/s -      2.805k in   5.022986s
```

```
$ ruby bench_json_generate.rb
Warming up --------------------------------------
                json    65.000  i/100ms
Calculating -------------------------------------
                json    659.576  (± 1.5%) i/s -      3.315k in   5.027127s
```

```
require 'json'
require 'benchmark/ips'

obj = []

1000.times do |i|
  obj << {
    "id" => i,
    :age => 42,
  }
end

Benchmark.ips do |x|
  x.report "json" do |iter|
    count = 0
    while count < iter
      JSON.generate(obj)
      count += 1
    end
  end
end
```

https://github.com/flori/json/commit/a73323dc5e