Initialize attributes of meta model roles when used from Perl 6

The meta model roles are written in NQP. When they are composed into a class
written in Perl 6, the list and hash attributes will not automatically get
initialized. So do it manually.

src/Perl6/Metamodel/AttributeContainer.nqp

@@ -9,6 +9,10 @@ role Perl6::Metamodel::AttributeContainer {
     # Adds an attribute.
     method add_attribute($obj, $meta_attr) {
         my $name := $meta_attr.name;
+        if nqp::isnull(%!attribute_lookup) {
+            @!attributes := nqp::list();
+            %!attribute_lookup := nqp::hash();
+        }
         if nqp::existskey(%!attribute_lookup, $name) {
             nqp::die("Package '" ~ self.name($obj) ~
                 "' already has an attribute named '$name'");

src/Perl6/Metamodel/BaseType.nqp

@@ -17,6 +17,7 @@ role Perl6::Metamodel::BaseType {
     # Our MRO is just that of base type.
     method mro($obj) {
         unless @!mro {
+            @!mro := nqp::list();
             @!mro[0] := $obj;
             for $!base_type.HOW.mro($!base_type) {
                 @!mro.push($_);