Make the method cache authoritative in some cases.

This makes it much cheaper for things that are doing can style checks,
which is the case for grammars without any actions, .? and so forth.
colomon++ for the profile that led to this.

src/Perl6/Metamodel/ClassHOW.pm

@@ -66,6 +66,10 @@ class Perl6::Metamodel::ClassHOW
     # and if it is calls $calculator with the object and method name to
     # calculate an invokable object.
     method add_fallback($obj, $condition, $calculator) {
+        # Adding a fallback means any method cache is no longer authoritative.
+        pir::set_method_cache_authoritativeness__vPi($obj, 0);
+
+        # Add it.
         my %desc;
         %desc<cond> := $condition;
         %desc<calc> := $calculator;
@@ -201,6 +205,7 @@ class Perl6::Metamodel::ClassHOW
     my $junction_type;
     my $junction_autothreader;
     method setup_junction_fallback($type, $autothreader) {
+        pir::set_method_cache_authoritativeness__vPi($type, 0);
         $junction_type := $type;
         $junction_autothreader := $autothreader;
     }
@@ -225,4 +230,9 @@ class Perl6::Metamodel::ClassHOW
         # Otherwise, didn't find anything.
         nqp::null()
     }
+
+    # Does the type have any fallbacks?
+    method has_fallbacks($obj) {
+        return nqp::istype($obj, $junction_type) || +@!fallbacks;
+    }
 }

src/Perl6/Metamodel/MROBasedMethodDispatch.pm

@@ -23,20 +23,27 @@ role Perl6::Metamodel::MROBasedMethodDispatch {
         # lower in the class hierarchy "shadowed" it.
         my %cache;
         my @mro_reversed;
+        my $authable := 1;
         for self.mro($obj) {
             @mro_reversed.unshift($_);
         }
         for @mro_reversed {
             for $_.HOW.method_table($_) {
                 %cache{$_.key} := $_.value;
             }
+            if nqp::can($_.HOW, 'is_composed') && !$_.HOW.is_composed($_) {
+                $authable := 0;
+            }
         }
 
         # Also add submethods.
         for $obj.HOW.submethod_table($obj) {
             %cache{$_.key} := $_.value;
         }
 
-        pir::publish_method_cache__0PP($obj, %cache)
+        pir::publish_method_cache__0PP($obj, %cache);
+        unless nqp::can(self, 'has_fallbacks') && self.has_fallbacks($obj) {
+            pir::set_method_cache_authoritativeness__0Pi($obj, $authable);
+        }
     }
 }

src/Perl6/Metamodel/MethodContainer.pm

@@ -11,6 +11,9 @@ role Perl6::Metamodel::MethodContainer {
 
     # Add a method.
     method add_method($obj, $name, $code_obj) {
+        # Adding a method means any cache is no longer authoritative.
+        pir::set_method_cache_authoritativeness__vPi($obj, 0);
+
         # Ensure we haven't already got it.
         if nqp::existskey(%!methods, $name) || nqp::existskey(%!submethods, $name) {
             nqp::die("Package '" ~ self.name($obj) ~ "' already has a method '" ~