add BytesCopy and StringCopy

[zhuzhu67373]

Bytes() and String() cannot be used when using fflib.Buffer in sync.Pool , like:

var bufPool = sync.Pool{
    New: func() interface{} {
        return fflib.NewBuffer([]byte{})
    },
}

func GetBuf() *fflib.Buffer {
    return bufPool.Get().(*fflib.Buffer)
}

func PutBuf(buf *fflib.Buffer) {
    buf.Reset()
    bufPool.Put(buf)
}

So I add copy version of Bytes() and String()