[swift-evolution] [pitch] Add a NativeType type alias to Int/UInt

[Martin R]

Some algorithms depend on the size of the `(U)Int` type. A typical example are hashing
functions, as in Hashing.swift (https://github.com/apple/swift/blob/master/stdlib/public/core/Hashing.swift#L142)

    func _mixInt(_ value: Int) -> Int {
    #if arch(i386) || arch(arm)
      return Int(_mixInt32(Int32(value)))
    #elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
      return Int(_mixInt64(Int64(value)))
    #endif
    }

I suggest to add a "NativeType" type alias to both Int and UInt. On a 64-bit platform this
would be

    extension Int {
        public typealias NativeType = Int64
    }
    extension UInt {
        public typealias NativeType = UInt64
    }

In the above example, the code would then simplify to

    func _mixInt(_ value: Int32) -> Int32 { return value }

    func _mixInt(_ value: Int64) -> Int64 { return value }


    func _mixInt(_ value: Int) -> Int {
        return Int(_mixInt(Int.NativeType(value)))
    }

i.e. one can write size-dependent code without using platform conditions (which have to be
maintained for new platforms).

Note that a similar property already exists for CGFloat:

    public struct CGFloat {

        /// The native type used to store the CGFloat, which is Float on
        /// 32-bit architectures and Double on 64-bit architectures.
        public typealias NativeType = Double
    
    }
    
Martin