fixup debugger files wrt to MaybeDangling

- fixup `BTreeMap` gdb provider
- fixup `ManuallyDrop` natvis thingy

Now that `MaybeUninit` contains `ManuallyDrop` which contains
`MaybeDangling` (tbc this is the addition), we need to unwrap one more
layer.

src/etc/gdb_providers.py

@@ -298,7 +298,7 @@ def children_of_btree_map(map):
 
         for i in xrange(0, length + 1):
             if height > 0:
-                child_ptr = edges[i]["value"]["value"]
+                child_ptr = edges[i]["value"]["value"][ZERO_FIELD]
                 for child in children_of_node(child_ptr, height - 1):
                     yield child
             if i < length:
@@ -306,12 +306,12 @@ def children_of_btree_map(map):
                 key_type_size = keys.type.sizeof
                 val_type_size = vals.type.sizeof
                 key = (
-                    keys[i]["value"]["value"]
+                    keys[i]["value"]["value"][ZERO_FIELD]
                     if key_type_size > 0
                     else gdb.parse_and_eval("()")
                 )
                 val = (
-                    vals[i]["value"]["value"]
+                    vals[i]["value"]["value"][ZERO_FIELD]
                     if val_type_size > 0
                     else gdb.parse_and_eval("()")
                 )

src/etc/natvis/libcore.natvis

@@ -35,9 +35,9 @@
   </Type>
 
   <Type Name="core::mem::manually_drop::ManuallyDrop&lt;*&gt;">
-    <DisplayString>{value}</DisplayString>
+    <DisplayString>{value.__0}</DisplayString>
     <Expand>
-      <ExpandedItem>value</ExpandedItem>
+      <ExpandedItem>value.__0</ExpandedItem>
     </Expand>
   </Type>