Smarter errors around `Function.length`

I don't know how often other people have encountered this, but here goes: In the same vein as our uncalled function checks, most usages of `Function.length` are _probably_ an error. Here's a toy example that just bit me: ```ts declare function getResult(): string[]; for (let i = 0; i< getResult.length; i++) { getResult().slice(0, i).map(x => console.log(x)); } ``` Minimally, I think we could suggest that if a function returns something with a `length` property, and the function's `length` property is inspected instead, we could probably mark it as suspect.