Use $/.from ~ ",' ~ $/.to as a POD block id when adding them to $=POD

The previous approach didn't work for two identical POD blocks existing
in the document, and caused a failure in t/pod/07-tables.t

src/Perl6/Actions.pm

@@ -229,7 +229,7 @@ class Perl6::Actions is HLL::Actions {
         my $child := $<pod_block>.ast;
         # make sure we don't push the same thing twice
         if $child {
-            my $id := nqp::where($child);
+            my $id := $/.from ~ "," ~ ~$/.to;
             if !$*POD_BLOCKS_SEEN{$id} {
                 $*POD_BLOCKS.push($child);
                 $*POD_BLOCKS_SEEN{$id} := 1;