Reimpl byte[] stream without synchronization

The byte[] output stream used here extended ByteArrayOutputStream
from the JDK, which sychronizes all mutation operations (like
writes). Since this is only going to be used once within a given
call stack, it needs no synchronization.

This change more than triples the performance of a benchmark of
dumping an array of empty arrays and should increase performance
of all dump forms.

java/src/json/ext/ByteListDirectOutputStream.java

@@ -3,14 +3,54 @@
 import org.jcodings.Encoding;
 import org.jruby.util.ByteList;
 
-import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.util.Arrays;
+
+public class ByteListDirectOutputStream extends OutputStream {
+    private byte[] buffer;
+    private int length;
 
-public class ByteListDirectOutputStream extends ByteArrayOutputStream {
     ByteListDirectOutputStream(int size) {
-        super(size);
+        buffer = new byte[size];
     }
 
     public ByteList toByteListDirect(Encoding encoding) {
-        return new ByteList(buf, 0, count, encoding, false);
+        return new ByteList(buffer, 0, length, encoding, false);
+    }
+
+    @Override
+    public void write(int b) throws IOException {
+        int myLength = this.length;
+        grow(this, buffer, myLength, 1);
+        buffer[length++] = (byte) b;
+    }
+
+    @Override
+    public void write(byte[] bytes, int start, int length) throws IOException {
+        int myLength = this.length;
+        grow(this, buffer, myLength, length);
+        System.arraycopy(bytes, start, buffer, myLength, length);
+        this.length = myLength + length;
+    }
+
+    @Override
+    public void write(byte[] bytes) throws IOException {
+        int myLength = this.length;
+        int moreLength = bytes.length;
+        grow(this, buffer, myLength, moreLength);
+        System.arraycopy(bytes, 0, buffer, myLength, moreLength);
+        this.length = myLength + moreLength;
+    }
+
+    private static void grow(ByteListDirectOutputStream self, byte[] buffer, int myLength, int more) {
+        int newLength = myLength + more;
+        int myCapacity = buffer.length;
+        int diff = newLength - myCapacity;
+        if (diff > 0) {
+            // grow to double current length or capacity + diff, whichever is greater
+            int growBy = Math.max(myLength, diff);
+            self.buffer = Arrays.copyOf(self.buffer, myCapacity + growBy);
+        }
     }
 }