Add laziness check in slow path

In the slow path, add laziness check. This fixes a bug with comparison
of lazy arrays where one is exhausted, and when the the number of elems
is compared a Failure occurs ("Lazy Lists Cannot .elems")

In order for the laziness check to work in instances where two identical
lazy lists are compared, modify "ZipIterable" so that all the sub-
iterators are pulled an equal number of times, even if one of the
earlier iterators pulled "IterationEnd"

src/core/List.pm6

@@ -1641,7 +1641,7 @@ proto sub prefix:<|>(|) {*}
 multi sub prefix:<|>(\x) { x.Slip }
 
 sub CMP-SLOW(@a, @b) {
-    (@a Zcmp @b).first(&prefix:<?>) || @a <=> @b
+    (@a Zcmp @b).first(&prefix:<?>) || &infix:<cmp>( |do .is-lazy for @a, @b ) || @a <=> @b
 }
 
 multi sub infix:<cmp>(@a, @b) {

src/core/Rakudo/Iterator.pm6

@@ -3844,18 +3844,21 @@ class Rakudo::Iterator {
                     nqp::stmts(
                       (my int $i = -1),
                       (my int $elems = nqp::elems($!iters)),
+                      (my int $is_iterend = 0),
                       (my $buf :=
                         nqp::setelems(nqp::create(IterationBuffer),$elems)),
-                      nqp::until(
-                        nqp::iseq_i(($i = nqp::add_i($i,1)),$elems)
-                         || nqp::eqaddr(
-                              (my $pulled := nqp::atpos($!iters,$i).pull-one),
-                              IterationEnd
-                            ),
-                        nqp::bindpos($buf,$i,$pulled)
+                      nqp::while(
+                        nqp::islt_i(($i = nqp::add_i($i,1)),$elems),
+                        nqp::if(
+                          nqp::eqaddr(
+                            (my $pulled := nqp::atpos($!iters,$i).pull-one), IterationEnd
+                          ),
+                          $is_iterend = 1,
+                          nqp::bindpos($buf,$i,$pulled)
+                        )
                       ),
                       nqp::if(
-                        nqp::islt_i($i,$elems),  # at least one exhausted
+                        $is_iterend,  # at least one exhausted
                         nqp::stmts(
 #?if jvm
                           ($!iters := Mu),
@@ -3930,18 +3933,21 @@ class Rakudo::Iterator {
                     nqp::stmts(
                       (my int $i = -1),
                       (my int $elems = nqp::elems($!iters)),
+                      (my int $is_iterend = 0),
                       (my $list :=
                         nqp::setelems(nqp::create(IterationBuffer),$elems)),
-                      nqp::until(
-                        nqp::iseq_i(($i = nqp::add_i($i,1)),$elems)
-                         || nqp::eqaddr(
-                              (my $pulled := nqp::atpos($!iters,$i).pull-one),
-                              IterationEnd
-                            ),
-                        nqp::bindpos($list,$i,$pulled)
+                      nqp::while(
+                        nqp::islt_i(($i = nqp::add_i($i,1)),$elems),
+                        nqp::if(
+                          nqp::eqaddr(
+                            (my $pulled := nqp::atpos($!iters,$i).pull-one), IterationEnd
+                          ),
+                          $is_iterend = 1,
+                          nqp::bindpos($list,$i,$pulled)
+                        )
                       ),
                       nqp::if(
-                        nqp::islt_i($i,$elems),  # at least one exhausted
+                        $is_iterend,  # at least one exhausted
                         nqp::stmts(
 #?if jvm
                           ($!iters := Mu),