reinstate new FALLBACK mechanism for type Failure

But since is meant to not trigger as a fallback for .CALL-ME and
.postcircumfix:<( )>, we need to add one of these on our own.

src/core/Failure.pm

@@ -37,15 +37,15 @@ my class Failure {
         self.exception.message ~ "\n" ~ self.backtrace;
     }
 
-    Failure.^add_fallback(
-        -> $, $ { True },
-        method ($name) {
-            $!exception.throw;
-        }
-    );
     method sink() {
         $!exception.throw($!backtrace) unless $!handled
     }
+    method CALL-ME(|) {
+        $!exception.throw($!backtrace)
+    }
+    method FALLBACK(*@) {
+        $!exception.throw($!backtrace)
+    }
 }
 
 proto sub fail(|) {*};