Replace precondition if with guard in NSNumber.encode.

Previously NSNumber.encode would call NSUnimplemented when you try to
do a keyed coding. Whilst it is true that this wasn't implemented it was
due to not being possible rather than not yet implemented.

With this patch we call preconditionFailure which is more consistent
with the rest of the project. We also use a guard so can remove a level of
indentation.

Author: Oliver Chick

Foundation/NSNumber.swift

@@ -609,35 +609,34 @@ open class NSNumber : NSValue {
     }
 
     open override func encode(with aCoder: NSCoder) {
-        if !aCoder.allowsKeyedCoding {
-            NSUnimplemented()
+        guard aCoder.allowsKeyedCoding else {
+            preconditionFailure("Unkeyed coding is unsupported.")
+        }
+        if let keyedCoder = aCoder as? NSKeyedArchiver {
+            keyedCoder._encodePropertyList(self)
         } else {
-            if let keyedCoder = aCoder as? NSKeyedArchiver {
-                keyedCoder._encodePropertyList(self)
+            if CFGetTypeID(self) == CFBooleanGetTypeID() {
+                aCoder.encode(boolValue, forKey: "NS.boolval")
             } else {
-                if CFGetTypeID(self) == CFBooleanGetTypeID() {
+                switch objCType.pointee {
+                case 0x42:
                     aCoder.encode(boolValue, forKey: "NS.boolval")
-                } else {
-                    switch objCType.pointee {
-                    case 0x42:
-                        aCoder.encode(boolValue, forKey: "NS.boolval")
-                        break
-                    case 0x63: fallthrough
-                    case 0x43: fallthrough
-                    case 0x73: fallthrough
-                    case 0x53: fallthrough
-                    case 0x69: fallthrough
-                    case 0x49: fallthrough
-                    case 0x6C: fallthrough
-                    case 0x4C: fallthrough
-                    case 0x71: fallthrough
-                    case 0x51:
-                        aCoder.encode(int64Value, forKey: "NS.intval")
-                    case 0x66: fallthrough
-                    case 0x64:
-                        aCoder.encode(doubleValue, forKey: "NS.dblval")
-                    default: break
-                    }
+                    break
+                case 0x63: fallthrough
+                case 0x43: fallthrough
+                case 0x73: fallthrough
+                case 0x53: fallthrough
+                case 0x69: fallthrough
+                case 0x49: fallthrough
+                case 0x6C: fallthrough
+                case 0x4C: fallthrough
+                case 0x71: fallthrough
+                case 0x51:
+                    aCoder.encode(int64Value, forKey: "NS.intval")
+                case 0x66: fallthrough
+                case 0x64:
+                    aCoder.encode(doubleValue, forKey: "NS.dblval")
+                default: break
                 }
             }
         }