Perl_doref(): make non-recursive

Perl_doref(): make non-recursive

This stops the following code from SEGVing for example:

    my $e = "\$r";
    $e = "+do{$e}" for 1..70_000;
    $e = "push \@{$e}, 1";
    eval $e;

Similarly with a long

    $a[0][0][0][0].....

This commit causes a slight change in behaviour, in that scalar(o)
is now only called once at the end of the top-level doref() call,
rather than at the end of processing each child. This should make no
functional difference, apart from speeding up compiling infinitesimally.