Improve Array#to_js speed

[krmbn0576]

Hi I am struggling with a large overhead when porting large sized arrays from Ruby to JS.
I found that this is caused by the implicit conversion Array#to_js and so I made this performance improvement.

measurement code below:

require "js"

JS.global[:console].time "old"
new_array = JS.eval("return []")
Array.new(10000).each do |element|
  new_array.push element
end
JS.global[:console].timeEnd "old" # 174.81494140625 ms

JS.global[:console].time "new"
new_array = JS.eval("return []")
Array.new(10000).each_slice(100) do |elements|
  new_array.push *elements
end
JS.global[:console].timeEnd "new" # 14.78466796875 ms

[kateinoigakukun]

Good finding, thank you! I slightly changed it to avoid unnecessary slicing. Confirmed it's still blazingly faster than the existing one.

old: 147.285ms
new: 6.113ms