Functions chapter

Some things to add to the current functions chapter:

- In the page on calling functions from functions, after showing what this looks like in various debuggers, I want to show what it looks like in a traceback. An exception should be raised in the innermost function (e.g. by intentionally misspelling a name) and then explain the resulting traceback.
- Scope and local variables:
- Explain that variables defined in one function are not accessible outside that function
- If they want to access them, they should return them
- However they can access global variables from within a function
- Explain that global and local variables should have different names to avoid errors
- Explain that `return` ends the function call. Sometimes beginners want to return a list of values like this:
```python
def double_nums(nums):
for n in nums:
return n * 2
```
Explain that they must build up a list instead. Maybe hint at generators but don't actually show them.

Things that will only be covered in a later chapter:

- Variadic, named, or optional arguments/parameters
- Higher order functions, functions as objects, functional programming, etc.
- How to set global variables with the `global` statement
- Inner functions and closures