Make .head(*-1)/.head(*-1) on native arrays faster

About 4x as fast for both.  This now also returns a native array of
the same type, rather than a Seq.

Also fix an off-by-one in .head(N).

src/core.c/native_array.pm6

@@ -38,16 +38,28 @@ my class array does Iterable does Positional {
 
     multi method head(array:D: Int:D $n) {
         my int $end = $n - 1;
-        nqp::isgt_i($end,nqp::elems(self))
+        nqp::isge_i($end,nqp::elems(self))
           ?? nqp::clone(self)
           !! nqp::slice(self,0,$end)
     }
+    multi method head(array:D: Callable:D $calculator) {
+        my int $end = $calculator(nqp::elems(self)) - 1;
+        nqp::islt_i($end,0)
+          ?? nqp::create(self.WHAT)
+          !! nqp::slice(self,0,$end)
+    }
     multi method tail(array:D: Int:D $n) {
         my int $start = nqp::elems(self) - $n;
         nqp::islt_i($start,0)
           ?? nqp::clone(self)
           !! nqp::slice(self,$start,nqp::sub_i(nqp::elems(self),1))
     }
+    multi method tail(array:D: Callable:D $calculator) {
+        my int $start = $calculator(nqp::elems(self));
+        nqp::isge_i($start,nqp::elems(self))
+          ?? nqp::create(self.WHAT)
+          !! nqp::slice(self,$start,nqp::sub_i(nqp::elems(self),1))
+    }
 
     sub INDEX_OUT_OF_RANGE(Int:D $got --> Nil) {
         X::OutOfRange.new(what => "Index", :$got, range => "0..^Inf").throw