Base64: Speedup encoding fix 3

- Use String(unsafeUninitializedCapacity:) instead of
  String(decoding:as:).

- This eliminates an excess String allocation and copy as the decoding
  of the input string to the result is eliminated.

Author: spevans

Sources/Foundation/NSData.swift

@@ -600,12 +600,13 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
         if dataLength == 0 { return "" }
 
         let capacity = estimateBase64Size(length: dataLength)
-        let ptr = UnsafeMutableRawPointer.allocate(byteCount: capacity, alignment: 4)
-        defer { ptr.deallocate() }
-        let buffer = UnsafeMutableRawBufferPointer(start: ptr, count: capacity)
-        let length = NSData.base64EncodeBytes(self, options: options, buffer: buffer)
+        let result = String(unsafeUninitializedCapacity: capacity) { bufferPtr in
+            let outputBuffer = UnsafeMutableRawBufferPointer(bufferPtr)
+            let length = NSData.base64EncodeBytes(self, options: options, buffer: outputBuffer)
+            return length
+        }
 
-        return String(decoding: UnsafeRawBufferPointer(start: ptr, count: length), as: Unicode.UTF8.self)
+        return result
     }
 
     /// Creates a Base64, UTF-8 encoded Data from the data object using the given options.