Undo special tests for index/rindex/indices

The test should really live in nqp::unbox_i, as mentioned in
http://irclog.perlgeek.de/perl6/2015-08-15#i_11061969

src/core/Cool.pm

@@ -196,10 +196,8 @@ my class Cool { # declared in BOOTSTRAP
 
     proto method indices(|) {*}
     multi method indices(Cool:D: Str(Cool) $needle, Int(Cool) $start = 0, :$overlap) {
-        my str $str  = nqp::unbox_s(self.Str);
-        return () if $start < 0 || $start > nqp::chars($str);
-
         my int $pos  = $start;
+        my str $str  = nqp::unbox_s(self.Str);
         my str $need = nqp::unbox_s($needle);
         my int $add  = $overlap ?? 1 !! nqp::chars($need) || 1;
 
@@ -220,10 +218,11 @@ my class Cool { # declared in BOOTSTRAP
         $i < 0 ?? Nil !! nqp::box_i($i,Int);
     }
     multi method index(Cool:D: Str(Cool) $needle, Int(Cool) $pos) {
-        my str $str = nqp::unbox_s(self.Str);
-        my int $i = $pos < 0 || $pos > nqp::chars($str)
-          ?? -1
-          !! nqp::index($str, nqp::unbox_s($needle), nqp::unbox_i($pos));
+        my int $i = nqp::index(
+          nqp::unbox_s(self.Str),
+          nqp::unbox_s($needle),
+          nqp::unbox_i($pos)
+        );
         $i < 0 ?? Nil !! nqp::box_i($i,Int);
     }
 
@@ -233,10 +232,11 @@ my class Cool { # declared in BOOTSTRAP
         $i < 0 ?? Nil !! nqp::box_i($i,Int);
     }
     multi method rindex(Cool:D: Str(Cool) $needle, Int(Cool) $pos) {
-        my str $str = nqp::unbox_s(self.Str);
-        my int $i = $pos < 0 || $pos > nqp::chars($str)
-          ?? -1
-          !! nqp::rindex($str, nqp::unbox_s($needle), nqp::unbox_i($pos));
+        my int $i = nqp::rindex(
+          nqp::unbox_s(self.Str),
+          nqp::unbox_s($needle),
+          nqp::unbox_i($pos)
+        );
         $i < 0 ?? Nil !! nqp::box_i($i,Int);
     }