Allow placeholder functions to compile

[andrewminer]

The various "placeholder" functions in mamba/init.py don't
actually match the signatures they will eventually have (e.g., they
don't accept a tag), and don't return values which can be used in a
with statement (i.e., no __enter__ or __exit__ methods). This
causes fatal errors when trying to load a *_spec.py file as a
standalone module.

This change stubs out the placeholder functions enough so that spec
files load properly without actually doing anything.

[andrewminer]

As a little more color on why I want this... I'm currently working on some code which uses introspection to serialize objects. I've got a bunch of test classes defined in my _spec.py which don't load properly because the "placeholder" functions never get replaced when loading the _spec.py file as a stand-alone module. This change makes it so that I can load the _spec.py file as a module, and therefore load that module as part of my test.