Use properties for all NSNumber transformations

The casts with `as` have worked up until now, but really this feels like
it's more explicit, more future proof, and possibly more efficient.

Argo/Types/StandardTypes.swift

@@ -32,7 +32,7 @@ extension Int: Decodable {
  */
  public static func decode(_ json: JSON) -> Decoded<Int> {
  switch json {
- case let .number(n): return pure(n as Int)
+ case let .number(n): return pure(n.intValue)
  default: return .typeMismatch(expected: "Int", actual: json)
  }
  }
@@ -116,7 +116,7 @@ extension Double: Decodable {
  */
  public static func decode(_ json: JSON) -> Decoded<Double> {
  switch json {
- case let .number(n): return pure(n as Double)
+ case let .number(n): return pure(n.doubleValue)
  default: return .typeMismatch(expected: "Double", actual: json)
  }
  }
@@ -155,7 +155,7 @@ extension Bool: Decodable {
  public static func decode(_ json: JSON) -> Decoded<Bool> {
  switch json {
  case let .bool(n): return pure(n)
- case let .number(n): return pure(n as Bool)
+ case let .number(n): return pure(n.boolValue)
  default: return .typeMismatch(expected: "Bool", actual: json)
  }
  }