fixup BTreeMap gdb provider

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

Author: WaffleLapkin

src/etc/gdb_providers.py

@@ -298,20 +298,20 @@ def cast_to_internal(node):
 
         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:
                 # Avoid "Cannot perform pointer math on incomplete type" on zero-sized arrays.
                 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("()")
                 )