plug a major leak of unhandled Failures

src/core/Failure.pm

@@ -14,6 +14,11 @@ my class Failure {
     # "Shouldn't happen."  We use note here because the dynamic scope in GC is likely meaningless.
     submethod DESTROY () { if not $!handled { note "WARNING: unhandled Failure detected in DESTROY:\n" ~ self.mess } }
 
+    # Turns out multidimensional lookups are one way to leak unhandled failures, so
+    # we'll just propagate the initial failure much as we propagate Nil on methods.
+    method AT-POS(|) { self }
+    method AT-KEY(|) { self }
+
     # TODO: should be Failure:D: multi just like method Bool,
     # but obscure problems prevent us from making Mu.defined
     # a multi. See http://irclog.perlgeek.de/perl6/2011-06-28#i_4016747