Move .^can into MROBasedMethodDispatch role.

Before, it was only implemented in ClassHOW, meaning that other things
that could do method dispatch missed out on having it. This fixes the
lack of .^can on EnumHOW, but also means others using the role will
get a .^can for free.

src/Perl6/Metamodel/ClassHOW.nqp

@@ -206,23 +206,7 @@ class Perl6::Metamodel::ClassHOW
     method is_composed($obj) {
         $!composed
     }
-
-    # Maybe this belongs on a role. Also, may be worth memoizing.
-    method can($obj, $name) {
-        my @meths;
-        my %smt := self.submethod_table($obj);
-        if nqp::existskey(%smt, $name) {
-            @meths.push(%smt{$name});
-        }
-        for self.mro($obj) {
-            my %mt := $_.HOW.method_table($_);
-            if nqp::existskey(%mt, $name) {
-                @meths.push(%mt{$name})
-            }
-        }
-        @meths
-    }
-
+
     # Stuff for junctiony dispatch fallback.
     my $junction_type;
     my $junction_autothreader;

src/Perl6/Metamodel/MROBasedMethodDispatch.nqp

@@ -32,7 +32,23 @@ role Perl6::Metamodel::MROBasedMethodDispatch {
             nqp::findmethod($qtype, $name)
         }
     }
-
+
+    # Maybe this belongs on a role. Also, may be worth memoizing.
+    method can($obj, $name) {
+        my @meths;
+        my %smt := self.submethod_table($obj);
+        if nqp::existskey(%smt, $name) {
+            @meths.push(%smt{$name});
+        }
+        for self.mro($obj) {
+            my %mt := $_.HOW.method_table($_);
+            if nqp::existskey(%mt, $name) {
+                @meths.push(%mt{$name})
+            }
+        }
+        @meths
+    }
+
     method publish_method_cache($obj) {
         # Walk MRO and add methods to cache, unless another method
         # lower in the class hierarchy "shadowed" it.