BaseDict._get_dict_or_value() recursively unwraps nested benedict
values with no cycle detection, so assigning a benedict into its own
nested descendant a second time recurses forever and crashes with
RecursionError. Track ids of mappings currently being unwrapped and
raise a clear ValueError as soon as a self-reference is detected,
instead of recursing into unrelated, equally unprotected traversal
code until the interpreter stack overflows.
dualfroz
88a0720c67