Improved compile-time validation for types expressible by integer literals

[xwu (Xiaodi Wu)]

@tanner0101

public struct MinuteInHour {
    let number: Int
    
    // not type-safe, does runtime checks
    public init(_ number: Int) {
        assert(number >= 0, "Minute cannot preceed 0")
        assert(number < 60, "Minute cannot exceed 60")
        self.number = number
    }
}

Better validation at compile time would benefit many types, not just those expressible by integer literals. The groundwork is being laid for this with compile-time constant expressions.

Once built out, one imagines that simply adding @compilerEvaluable to the initializer would get you the diagnostics you want. I don't see a role for additional protocols.