[perf] JSON encoding can be faster by skipping string building.

I realized that one of the test related to CharacterSet was very slow in
constrained devices, but also relatively slow in beefy Linux machines
(taking a couple of seconds to perform back and forth JSON
encoding/decoding trips in memory). The test in question have to embed
relatively large Data in the JSON string (around 128KB, if I remember
correctly), which are serialized as Base64.

When serializing the data, the serializer ask for the String
representation of each object, and then appends all of them into a
String, to later transform into an UTF-8 C string copied into a Data.
One can improve performance by appending the UTF-8 representation of
each piece of the JSON serialization instead of the String building
code, since appending [UInt8] without doing all the checking that String
will do is faster for this case.

Author: drodriguez

Foundation/JSONSerialization.swift

@@ -123,14 +123,14 @@ open class JSONSerialization : NSObject {
     /* Generate JSON data from a Foundation object. If the object will not produce valid JSON then an exception will be thrown. Setting the NSJSONWritingPrettyPrinted option will generate JSON with whitespace designed to make the output more readable. If that option is not set, the most compact possible JSON will be generated. If an error occurs, the error parameter will be set and the return value will be nil. The resulting data is a encoded in UTF-8.
      */
     internal class func _data(withJSONObject value: Any, options opt: WritingOptions, stream: Bool) throws -> Data {
-        var jsonStr = String()
+        var jsonStr = [UInt8]()
 
         var writer = JSONWriter(
             pretty: opt.contains(.prettyPrinted),
             sortedKeys: opt.contains(.sortedKeys),
             writer: { (str: String?) in
                 if let str = str {
-                    jsonStr.append(str)
+                    jsonStr.append(contentsOf: Array(str.utf8))
                 }
             }
         )
@@ -147,10 +147,8 @@ open class JSONSerialization : NSObject {
             fatalError("Top-level object was not NSArray or NSDictionary") // This is a fatal error in objective-c too (it is an NSInvalidArgumentException)
         }
 
-        let count = jsonStr.utf8.count
-        return jsonStr.withCString {
-            Data(bytes: $0, count: count)
-        }
+        let count = jsonStr.count
+        return Data(bytes: &jsonStr, count: count)
     }
 
     open class func data(withJSONObject value: Any, options opt: WritingOptions = []) throws -> Data {