xo-gc stack: refactor + streamline.

Retiring unused Collector typealiases.
Fix #include topology.
Fix/improve write barrier setup.

idl/Collector.json5

@@ -4,7 +4,7 @@
     output_hpp_dir: "include/xo/alloc2",
     output_impl_subdir: "gc",
     includes: [
-        "<xo/alloc2/Allocator.hpp>",
+        "<xo/alloc2/Allocator_basic.hpp>",
         "<xo/alloc2/Generation.hpp>",
         "<xo/alloc2/role.hpp>",
 //        "<cstdint>",
@@ -254,6 +254,7 @@
             name: "assign_member",
             doc: [
                 "Assign pointer @p p_lhs to destination @p rhs, within parent allocation @p parent",
+                "DEPRECATED. Only used in MockCollector for gc unit test",
                 "",
                 "Require: gc not in progress",
             ],

idl/GCObject.json5

@@ -4,8 +4,8 @@
     output_hpp_dir: "include/xo/alloc2",
     output_impl_subdir: "gc",
     includes: [
-        "<xo/alloc2/Allocator.hpp>",
-        "<xo/alloc2/Collector.hpp>",
+        "<xo/alloc2/Allocator_basic.hpp>",
+//        "<xo/alloc2/Collector.hpp>",
         "<xo/alloc2/GCObjectVisitor.hpp>",
         "<cstdint>",
         "<cstddef>",
@@ -38,11 +38,11 @@
             doc: ["fomo allocator type"],
             definition: "xo::mm::AAllocator",
         },
-        {
-            name: "ACollector",
-            doc: ["fomo collector type"],
-            definition: "xo::mm::ACollector",
-        },
+//        {
+//            name: "ACollector",
+//            doc: ["fomo collector type"],
+//            definition: "xo::mm::ACollector",
+//        },
         {
             name: "AGCObjectVisitor",
             doc: ["fomo collector type"],