Compatible Array#flatten

Array#flatten with depth argument is useful.

[[1], [2], [[3], 4]].flatten(1)
#=> [1, 2, [3], 4]

That's been available since ruby 1.8.7, but older ruby such as ruby 1.8.6 doesn't support it.

Here is the pure-ruby implementation of Array#flatten. Use it in case.

I used it for my library spawn-for-legacy to support ruby 1.8.6.

Rails programmers don't need this code. Rails now only supports ruby newer than 1.8.7.