[Pitch] Dependent Integer Expressions in Generic Arguments

Hey all, I've been working on several small prototypes of this ever since SE-0452 originally introduced integer generic parameters and mentioned arithmetic generics in the "Future directions" section. Since SE-0531 (literal expressions in integer generic argument positions) got accepted recently too, I decided to make another prototype and got far enough this time that I wanted to pitch it for real. Basically, Swift still cannot use arithmetic with integer generic parameters in generic argumen...