Unpickler.load_inst(), Unpickler.load_obj(), Unpickler.load_build():

Fixed problems when unpickling in restricted execution environments.
These methods try to assign to an instance's __class__ attribute, or
access the instances __dict__, which are prohibited in REE.  For the
first two methods, I re-implemented the old behavior when assignment
to value.__class__ fails.

For the load_build() I also re-implemented the old behavior when
inst.__dict__.update() fails but this means that unpickling in REE is
semantically different than unpickling in unrestricted mode.