Create a scaffold for lessons delving deeper into functions

[willingc]

Functions are a broad subject in development. There are a lot of things that folks learn over time.

Let's begin working on a scaffold of important topics for learner progression.

Leave thoughts of topics here.

See discussion in #58 and #67 for more ideas.

[sneakers-the-rat]

off the top of my head:

• type annotations (not function-specific but i have to write this so my fingers stop screaming at me to type it)
• *args, **kwargs (and when to use them)
• generators
• context managers
• decorators
• async
• closures
• currying/functools.partial
• lambdas
• descriptors/dynamic binding functions -> instance methods

on more of the "how-to/best practices/software design" side of things...

• pure functions/stateful functions/idempotency, maybe in the context of concurrency?
• functional programming in python - map/filter/itertools
• operators as functions (the operator module)
• module-level dunder methods - modules as classes
• wrapping and metaprogramming
• caching with lru_cache as a way to introduce hashable vs. unhashable and why that's important
• "how do pytest fixtures work" as like a worked example of advanced metaprogramming with functions
• "functions are objects" and the python data and execution model

cruising around the stdlib and language reference docs always make me with the python core devs accepted my PR to make the docs more navigable, but alas, i unfortunately had the audacity to turn issue number 123456 into a shitpost at the same time and so i think it's forever doomed to purgatory.