Don't use .push in Buf.list when we already know exactly what index w…

…e're going to use.

src/core/Buf.pm

@@ -42,7 +42,7 @@ class Buf does Positional {
         my int $bytes = nqp::elems($!buffer);
         my int $i = 0;
         while $i < $bytes {
-            @l.push: nqp::p6box_i(nqp::atpos_i($!buffer, $i));
+            @l[$i] = nqp::p6box_i(nqp::atpos_i($!buffer, $i));
             $i = $i + 1;
         }
         @l;