None unification

[ddfisher]

Currently, None types in the code either mean:

• the internal Void type: a type that cannot be used
• None: an uninhabited type (i.e. a type without any values)
• the internal NoneTyp type: the type that contains only the value None (this cannot currently be explicitly specified)

After doing proper Optional checking, I propose that we make None types always mean NoneTyp. Optional checking will prevent functions that return None from being used in places they shouldn't, and allowing users to specify something is the uninhabited type isn't currently useful at all.

[ddfisher]

Doing this should fix #299.

[gvanrossum]

Marked as needs discussion since it would be good to understand the consequences more precisely.

[ilevkivskyi]

There is no more Void, and None always means NoneTyp() so I think this can be closed.