Refactor forward_list

[Paolo Carlini]

Ed Smith-Rowland wrote:
> Greetings,
>
> In forward_list factored out some routines that construct sequences of
> nodes.
>
> Added some template technology to prevent (int, int, *) in ctors,
> insert, and merge from getting sent to (Iter, Iter, *) routines.
Excellent, applied (besides the new testcase which already exists in the
headers/forward_list subdir).

Can you add the dispatching machinery to assign too?

Thanks,
Paolo.