Make @A[Range] about 3x faster

- for completely refied list/arrays only
- without any adverbs
- by adding a highly specialized postcircumfix:<[ ](List:D, Range:D) candidate

src/core/array_slice.pm

@@ -209,6 +209,51 @@ multi sub postcircumfix:<[ ]>( \SELF, Any:D \pos, :$v!, *%other ) is raw {
 }
 
 # @a[@i]
+multi sub postcircumfix:<[ ]>(List:D \SELF, Range:D \range, *%_ ) is raw {
+    nqp::if(
+      nqp::iscont(range),
+      SELF.AT-POS(range.Int),                    # don't iterate cause itemized
+      nqp::if(                                   # iterate
+        nqp::elems(nqp::getattr(%_,Map,'$!storage')),
+        postcircumfix:<[ ]>(SELF, range.list, |%_),             # named params
+        nqp::if(                                 # no named params
+          nqp::getattr(SELF,List,'$!todo')
+            || nqp::not_i(nqp::getattr(range,Range,'$!is-int'))
+            || nqp::isfalse(my $reified := nqp::getattr(SELF,List,'$!reified')),
+          POSITIONS(SELF, range).map({ SELF[$_] }).eager.list,  # too difficult
+          nqp::stmts(                            # simple reified
+            (range.int-bounds(my int $pos, my int $last)),
+            nqp::if(
+              nqp::islt_i($pos,0),
+              Failure.new(X::OutOfRange.new(     # start out of range
+                :what($*INDEX // 'Index'), :got($pos), :range<0..^Inf>)),
+              nqp::stmts(                        # start point ok
+                (my $buffer := nqp::setelems(
+                  nqp::create(IterationBuffer),
+                  nqp::add_i(nqp::sub_i($last,$pos),1)
+                )),
+                ($pos = nqp::sub_i($pos,1)),
+                (my int $i = -1),
+                nqp::while(                      # fill the buffer
+                  nqp::isle_i(($pos = nqp::add_i($pos,1)),$last),
+                  nqp::bindpos(
+                    $buffer,
+                    ($i = nqp::add_i($i,1)),
+                    nqp::ifnull(
+                      nqp::atpos($reified,$pos),
+                      SELF.AT-POS($pos)
+                    )
+                  )
+                ),
+                nqp::p6bindattrinvres(           # make it a List  
+                  nqp::create(List),List,'$!reified',$buffer)
+              )
+            )
+          )
+        )
+      )
+    )
+}
 multi sub postcircumfix:<[ ]>( \SELF, Iterable:D \pos ) is raw {
     nqp::iscont(pos)
       ?? SELF.AT-POS(pos.Int)