Change RHooks() interface to not require a 'rexec' instance argument;
added set_rexec() method instead (which must be called by the RExec
instance using this RHooks instance).

Support dynamic loading of modules, at least for those modules that
are ok built-in modules.  Added new interfaces set_trusted_path() and
load_dynamic() to RExec class (the default trusted path consists of
all absolute pathnames in sys.path).

Change copy_except() to actually try to delete the exceptions.