Turn candidates_matching into cando.

Turns out we had a non-spec method that was really close to doing what
a spec'd method needed. Small refactor, including to the thing that
depended on it.

src/core/Main.pm

@@ -124,7 +124,7 @@ my sub MAIN_HELPER($retval = 0) is hidden_from_backtrace {
     my $?USAGE = gen-usage();
 
     # Get a list of candidates that match according to the dispatcher
-    my @matching_candidates = $m.candidates_matching(|@($p), |%($n));
+    my @matching_candidates = $m.cando(Capture.new(list => $p, hash => $n));
     # Sort out all that would fail due to binding
     @matching_candidates .=grep: {!has-unexpected-named-arguments($_.signature, $n)};
     # If there are still some candidates left, try to dispatch to MAIN

src/core/Routine.pm

@@ -25,7 +25,7 @@ my class Routine {
             (self,)
     }
 
-    method candidates_matching(|c) {
+    method cando(Capture $c) {
         my $disp;
         if self.is_dispatcher {
             $disp := self;
@@ -39,7 +39,7 @@ my class Routine {
             my Mu $cap := pir::find_lex__Ps('call_sig');
             pir::perl6ize_type__PP(pir::perl6_get_matching_multis__PPP($disp, $cap))
         }
-        checker(|c);
+        checker(|$c);
     }
 
     method multi() {