Make InfiniInt<T> generic over Element

[oscbyspro]

The current InfiniInt<T> is generic over a proxy type, rather than its element type, to satisfy the requirement machine:

struct InfiniInt<Source: SystemsInteger>: ArbitraryInteger {
    typealias Element =  Source.Element
}

I would prefer it being generic over its element type, but the compiler burns the world to ashes when I try. I'm unsure why:

struct InfiniInt<Element: SystemsInteger>: ArbitraryInteger { }

[oscbyspro]

Wow. Slava Pestov's fake protocol trick is magic ✨