Allow for initializations of "is List" attributes

This commit makes it possible to assign to attributes marked as "is List",
so that you can create Listy attributes that are immutable.

    class A {
        has @.foo is List
    }
    my $a = A.new( foo => (1,2,3) );
    dd $a;  # A.new(foo => (1, 2, 3))
    $a.a[1] = 666;  # cannot change immutable List

Adapt the workaround message for R#1226.

src/Perl6/Metamodel/BUILDPLAN.nqp

@@ -50,9 +50,11 @@ role Perl6::Metamodel::BUILDPLAN {
                               workaround =>
                                 "Create/Adapt TWEAK method in class "
                                   ~ $obj.HOW.name($obj)
-                                  ~ ", e.g:\n    method TWEAK(:"
+                                  ~ ", e.g:\n\n    method TWEAK() \{\n        "
                                   ~ $_.name
-                                  ~ ' = (initial values)) { }'
+                                  ~ " := (initial values) unless "
+                                  ~ $_.name
+                                  ~ ";\n    }"
                             ).throw;
                         }
                     }

src/Perl6/World.nqp

@@ -3285,11 +3285,16 @@ class Perl6::World is HLL::World {
 
                             my $sigil := nqp::substr(nqp::atpos($task,2),0,1);
 
-# nqp::getattr(self,Foo,'$!a').STORE(%init.AT-KEY('a'))
+# nqp::getattr(self,Foo,'$!a').STORE(%init.AT-KEY('a'), :initialize)
                             if $sigil eq '@' || $sigil eq '%' {
                                 $if.push(
                                   QAST::Op.new( :op<callmethod>, :name<STORE>,
-                                    $getattr, $value
+                                    $getattr, $value, QAST::WVal.new(
+                                      :value($!w.find_symbol(
+                                        ['Bool','True'], :setting-only
+                                      )),
+                                      :named('initialize')
+                                    )
                                   )
                                 );
                             }
@@ -3374,11 +3379,16 @@ class Perl6::World is HLL::World {
                               !! QAST::WVal.new(:value(nqp::atpos($task,3)));
 
                             my $sigil := nqp::substr(nqp::atpos($task,2),0,1);
-# nqp::getattr(self,Foo,'$!a').STORE($code(self,nqp::getattr(self,Foo,'$!a')))
+# nqp::getattr(self,Foo,'$!a').STORE($code(self,nqp::getattr(self,Foo,'$!a')), :initialize)
                             if $sigil eq '@' || $sigil eq '%' {
                                 $unless.push(
                                   QAST::Op.new( :op<callmethod>, :name<STORE>,
-                                    $getattr, $initializer
+                                    $getattr, $initializer, QAST::WVal.new(
+                                      :value($!w.find_symbol(
+                                        ['Bool','True'], :setting-only
+                                      )),
+                                      :named('initialize')
+                                    )
                                   )
                                 );
                             }