o Registries get their state restored from the restorer when EvalException begins
executing, and their state changes are maintained by the restorer
o StackedObjectProxy's _push/pop_object will no-op in that context
o made get_saved_proxied_obj handle proxies having their objects de-registered
within EvalException

o get_saved_proxied_obj needed to delve down to allow access to older
(overlaid) RegistryManagers' objects (test_restorer_nested_middleman)
o save_registry_state needed to save all state, so restoration can get to every
overlaid RegistryManagers' objects
(test_restorer_middlemen_nested_evalexception). cleaned it up to do all the
saving at once, instead of piecemeal

EvalException is enabled and RegistryManager and or EvalException detect an
unexpected exception, they'll call StackedObjectRestorer to:
o save the Registry state
o change currently registered StackedObjectProxies to detect when they're being
used within the interactive debugger via a threadlocal (resulting in a small
performance hit). They'll get their proxied object from the restorer when they
detect the presence of the debugger, otherwise they'll continue to work as
normal