Implement late-binding loop check

[Zac-HD]

Closes #263.

• Avoid duplicate warnings when inside nested loops (i.e. dedupe before appending)
• Only warn on names used in a Load context, e.g. ... for _ in ... is OK in a function, even if _ is also a loop variable
• Assigning to attributes or indexed items should not be considered an access, e.g. self.count += 1 does not make self a loop variable. I'm generally fine ruling that detecting bugs around attributes is out of scope, but the false-alarms need to be fixed

I've run this over our codebase at work (the motivating example 😅), and there are only a few false-positives where the function is used and then discarded within the loop iteration that defines it. These cases are also infeasible to reliably detect, so I'm comfortable leaving them to be handled by either manual var=var capture or # noqa comments.

[Zac-HD]

Ping @cooperlees?

[cooperlees]

Sorry about the delay. Been busy.

Makes sense to me. Includes Carl's suggestion + well documented + tested

• Plus links to a great doc for people to read to fully understand (like I did)

I feel this is going to find a lot of cases doing this. So lets release this on it's own and wreak havoc.

[Zac-HD]

No worries, life happens! Bring on the havoc, we'll see if people are complaining about false-alarms or just alarms they don't like 😁