Fix mysterious "Seq already iterated" errors for Seq containing Seqs

When a Seq contains another Seq, the latter often throwed errors about
having already been iterated even if the code (like List.join) really
only iterated once.

The reason is that a Seq when sunk will iterate and sink all its items.
We need to take care whenever we push values while iterating because
push returns the pused value and thus may .sink it.

Fixes for example ('ABC'.ords xx 1).join

Regressions in S05-modifier/counted.t and counted-match.t but I really
have no idea how that could happen.

src/core/Iterator.pm

@@ -26,7 +26,7 @@ my role Iterator {
         while $i < $n {
             $pulled := self.pull-one();
             last if $pulled =:= IterationEnd;
-            $target.push($pulled);
+            my $ = $target.push($pulled); # we may not .sink $pulled here, since it can be a Seq
             $i = $i + 1;
         }
         $pulled =:= IterationEnd

src/core/List.pm

@@ -107,7 +107,7 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
                     }
                 }
                 else {
-                    $!reification-target.push(current);
+                    my $ = $!reification-target.push(current);
                 }
             }
             nqp::elems($!reified);
@@ -130,7 +130,7 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
                         }
                     }
                     else {
-                        $!reification-target.push(current);
+                        my $ = $!reification-target.push(current);
                     }
                 }
                 $!future := Mu unless nqp::elems($!future);
@@ -148,7 +148,7 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
                     my \current = nqp::shift($!future);
                     nqp::istype(current, Slip) && nqp::isconcrete(current)
                         ?? current.iterator.push-all($!reification-target)
-                        !! $!reification-target.push(current);
+                        !! my $ = $!reification-target.push(current);
                 }
                 $!future := Mu;
             }
@@ -198,7 +198,7 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
         my int $n = nqp::elems(vm-tuple);
         while $i < $n {
             my \consider = nqp::atpos(vm-tuple, $i);
-            nqp::push(future, nqp::iscont(consider)
+            my $ = nqp::push(future, nqp::iscont(consider)
                 ?? consider
                 !! nqp::istype(consider, Iterable)
                     ?? consider.flat.Slip
@@ -223,7 +223,7 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
         my \iterbuffer = IterationBuffer.CREATE;
         nqp::bindattr(list, List, '$!reified', iterbuffer);
         for @things {
-            iterbuffer.push($_);
+            my $ = iterbuffer.push($_);
         }
         list
     }
@@ -348,7 +348,7 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
                     !! nqp::elems($!reified);
                 my int $i = $!i;
                 while $i < $n {
-                    $target.push(nqp::ifnull(nqp::atpos($!reified, $i), Any));
+                    my $ = $target.push(nqp::ifnull(nqp::atpos($!reified, $i), Any));
                     $i = $i + 1;
                 }
                 $!i = $n;

src/core/Seq.pm

@@ -315,7 +315,7 @@ sub GATHER(&block) {
                             iter!start-slip-wanted($taken),
                             ($wanted = nqp::getattr_i(iter, self, '$!wanted'))),
                         nqp::stmts(
-                            nqp::getattr(iter, self, '$!push-target').push($taken),
+                            my $ = nqp::getattr(iter, self, '$!push-target').push($taken),
                             ($wanted = nqp::bindattr_i(iter, self, '$!wanted',
                                 nqp::sub_i(nqp::getattr_i(iter, self, '$!wanted'), 1))))),
                     nqp::if(nqp::iseq_i($wanted, 0),
@@ -326,7 +326,7 @@ sub GATHER(&block) {
                 )
             }
             nqp::bindattr(iter, self, '&!resumption', {
-                nqp::handle(&block(), 'TAKE', $taker());
+                my $ = nqp::handle(&block(), 'TAKE', $taker());
                 nqp::continuationcontrol(0, PROMPT, -> | {
                     nqp::bindattr(iter, self, '&!resumption', Callable)
                 });
@@ -368,12 +368,12 @@ sub GATHER(&block) {
         method !start-slip-wanted(\slip) {
             my $value := self.start-slip(slip);
             unless $value =:= IterationEnd {
-                $!push-target.push($value);
+                my $ = $!push-target.push($value);
                 my int $i = 1;
                 my int $n = $!wanted;
                 while $i < $n {
                     last if ($value := self.slip-one()) =:= IterationEnd;
-                    $!push-target.push($value);
+                    my $ = $!push-target.push($value);
                     $i = $i + 1;
                 }
                 $!wanted = $!wanted - $i;
@@ -386,7 +386,7 @@ sub GATHER(&block) {
             my $value;
             while $i < $n {
                 last if ($value := self.slip-one()) =:= IterationEnd;
-                $!push-target.push($value);
+                my $ = $!push-target.push($value);
                 $i = $i + 1;
             }
             $!wanted = $!wanted - $i;

src/core/control.pm

@@ -68,7 +68,10 @@ my &take := -> | {
 #?if !jvm
 proto sub take(|) { * }
 multi sub take()   { die "take without parameters doesn't make sense" }
-multi sub take(\x) { THROW(nqp::const::CONTROL_TAKE,nqp::p6recont_ro(x)); x }
+multi sub take(\x) {
+    my $ = THROW(nqp::const::CONTROL_TAKE, nqp::p6recont_ro(x));
+    x
+}
 multi sub take(|) {
     my $list := RETURN-LIST(nqp::p6argvmarray());
     THROW( nqp::const::CONTROL_TAKE, nqp::p6recont_ro($list) );