Implement IO::CatHandle.handles

[zoffixznet]

Looking at RT#132885, it seems like it could be handy to have IO::CatHandle.handles method (or io-handles to differentiate from handles the verb) that simply returns a Seq of the handles. Starting with the $!active-handle and calling .next-handle on each .pull-one. Then the code in that ticket could be just written like:

    for $*ARGFILES.handles -> $fh {
        put "NEWFILE:";
        put "$fh.path():$(++$) $_" for $fh.lines: 3;
    }