Introduce :is-lazy named for gather { } blocks

After this commit, you can indicate that your `gather` block is to
be treated as a lazy one:

    gather {
        ...
    } :is-lazy;

src/core.c/Rakudo/Iterator.pm6

@@ -1858,6 +1858,8 @@ class Rakudo::Iterator {
               $n
             )
         }
+
+        method is-lazy() { $!is-lazy }
     }
     method Gather(&block, :$is-lazy) { Gather.new(&block, $is-lazy.Bool) }
 

src/core.c/Seq.pm6

@@ -178,7 +178,9 @@ my class Seq is Cool does Iterable does Sequence {
     }
 }
 
-sub GATHER(&block) { Seq.new(Rakudo::Iterator.Gather(&block)) }
+sub GATHER(&block, :$is-lazy) {
+    Seq.new(Rakudo::Iterator.Gather(&block, :$is-lazy))
+}
 
 multi sub infix:<eqv>(Seq:D \a, Seq:D \b) {
     nqp::hllbool(