Fix a whole ton of methods on cached Seqs

When Seq is cached, its $!list gets .DEFINITE and we can get
cached stuff from it. However, several methods do not check
for presence of $!list cache and assume the absence of $!iter means
the Seq has been consumed and there is no cache.

Fixes incorrect "Seq has been consumed; try caching" error for:
infix:<eqv>, .iterator, .Slip, .join, .List, .list, .eager,
.Array, .is-lazy, and .sink

Bug find: https://irclog.perlgeek.de/perl6-dev/2017-05-03#i_14524925

src/core/Seq.pm

@@ -56,15 +56,23 @@ my class Seq is Cool does Iterable does PositionalBindFailover {
             ($!iter := Iterator),
             iter
           ),
-          X::Seq::Consumed.new.throw
+          nqp::if(
+            $!list.DEFINITE,
+            $!list.iterator,
+            X::Seq::Consumed.new.throw
+          )
         )
     }
 
     multi method is-lazy(Seq:D:) {
         nqp::if(
           $!iter.DEFINITE,
           $!iter.is-lazy,
-          X::Seq::Consumed.new.throw
+          nqp::if(
+            $!list.DEFINITE,
+            $!list.is-lazy,
+            X::Seq::Consumed.new.throw
+          )
         )
     }
 
@@ -192,6 +200,10 @@ my class Seq is Cool does Iterable does PositionalBindFailover {
           nqp::stmts(
             $!iter.sink-all,
             ($!iter := Iterator)
+          ),
+          nqp::if(
+            $!list.DEFINITE,
+            $!list.iterator.sink-all
           )
         )
     }