Function definitions don't use the proper namespace in the IPython shell

For instance, if you run

```
a = 1
```

and

```
def test():
print(a)

test()
```

in the IPython shell, it gives `NameError: name 'a' is not defined`.

The use of SetPropagatingDict would fix this. It looks like the IPython shell is doing some weird stuff, so I'm not exactly clear on how to fix this.

Also we should update the docs for SetPropagatingDict to make it clear that basically every shell should use it, unless they really know what they are doing. This is because globals/locals being separate dictionaries creates odd behavior with exec (basically, it acts like a class body), which is generally much different from what you would expect from a shell.

Reproduce the IPython shell example with a global variable referenced inside a function, then inspect the shell execution path and the mentioned SetPropagatingDict behavior. Done means the example resolves the variable correctly and the SetPropagatingDict documentation explains why shells should generally use it.

Written by the indexing model from the issue text.