Implement WeakKeyMap on VWA

Benchmark:

```
puts(Benchmark.measure do
  10_000_000.times do
    ObjectSpace::WeakKeyMap.new
  end
end)
```

Before:

```
2.554772   0.014167   2.568939 (  2.575763)
```

After:

```
1.994920   0.013583   2.008503 (  2.012139)
```

1 files changed, 2 insertions, 3 deletions

diff --git a/weakmap.c b/weakmap.c
index 67be955fe9..6d66181ffc 100644
--- a/weakmap.c
+++ b/weakmap.c
@@ -557,7 +557,6 @@ wkmap_free(void *ptr)
 
     st_foreach(w->table, wkmap_free_table_i, 0);
     st_free_table(w->table);
-    xfree(w);
 }
 
 static size_t
@@ -565,7 +564,7 @@ wkmap_memsize(const void *ptr)
 {
     const struct weakkeymap *w = ptr;
 
-    size_t size = sizeof(*w);
+    size_t size = 0;
     size += st_memsize(w->table);
     /* Each key of the table takes sizeof(VALUE) in size. */
     size += st_table_size(w->table) * sizeof(VALUE);
@@ -619,7 +618,7 @@ static const rb_data_type_t weakkeymap_type = {
         wkmap_memsize,
         wkmap_compact,
     },
-    0, 0, RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED
+    0, 0, RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_EMBEDDABLE
 };
 
 static int