export forder

[jangorecki]

so it can be legally used outside of [.data.table

[mattdowle]

Discussion here: #3817 (comment)

and move colnamesInt call down to C: https://github.com/Rdatatable/data.table/pull/3817/files#r320951619

[akersting]

I would also like to have forder and forderv exported. These are very useful functions also for other packages.

[mattfidler]

I think you upvote on the top to show your interest

[jangorecki]

Before exporting, we should resolve #4467, also lazy argument may need some rework because its current API is more internal-like.