Expose new R:I:While/Until in .head/.tail

- .head taking a Callable:D: block should return trueish while values
  should be passed on.  The first falsish value will end the iterator.
- .tail taking a Callable:D: block should return falsish while values
  are being skipped.  The first trueish value will start passing on
  values until iterator is exhausted.
- no breakage in spectest seen.

Please note that I feel these semantics are a bit in conflict with the
"* - x" semantics meaning "so many from end".  So this would be the first
case where a WhateverCode has a different semantic from a generic Callable.
Perhaps we should expose this functionality in separate .while/.until
methods.  Please discuss :-)

src/core/Any-iterable-methods.pm

@@ -1823,6 +1823,9 @@ Did you mean to add a stub (\{...\}) or did you mean to .classify?"
            Rakudo::Iterator.AllButLastNValues(self.iterator,-($w(0).Int))
         )
     }
+    multi method head(Any:D: Callable:D $cond) {
+        Seq.new(Rakudo::Iterator.While(self.iterator,$cond))
+    }
     multi method head(Any:D: $n) {
         Seq.new(Rakudo::Iterator.NextNValues(self.iterator,$n))
     }
@@ -1838,19 +1841,24 @@ Did you mean to add a stub (\{...\}) or did you mean to .classify?"
           $pulled
         )
     }
-    multi method tail(Any:D: $n) {
+    multi method tail(Any:D: WhateverCode $w) {
         Seq.new(
           nqp::if(
-            nqp::istype($n,WhateverCode)
-              && nqp::isgt_i((my $skip := -($n(0).Int)),0),
+            nqp::isgt_i((my $skip := -($w(0).Int)),0),
             nqp::stmts(
               (my $iterator := self.iterator).skip-at-least($skip),
               $iterator
             ),
-            Rakudo::Iterator.LastNValues(self.iterator,$n,'tail')
+            self.iterator
           )
         )
     }
+    multi method tail(Any:D: Callable:D $w) {
+        Seq.new(Rakudo::Iterator.Until(self.iterator,$w))
+    }
+    multi method tail(Any:D: $n) {
+        Seq.new(Rakudo::Iterator.LastNValues(self.iterator,$n,'tail'))
+    }
 
     proto method minpairs(|) {*}
     multi method minpairs(Any:D:) {