`static for` and `static if` to manipulate variadic generics

[IndianBoy42]

Have you read the proposal and discussion here?

Variadic generics design sketch

Variadic generics are a long-desired feature in Rust, but the syntax and semantics are not easy to get right. I've sketched out a possible design in a HackMD: I would appreciate feedback, especially on marked TODOs. Feel free to respond either here, or in comments on the HackMD.