Add functions for stream encoding/decoding of big strings

[xakep666]

This may be useful i.e. when your service should download file and send it inside json to other service without storing whole file in memory.
In my case service downloads a file from S3-like service and makes request to our email provider to send it as attachment (base64 encoded).
Currently I'm using this snippet:

func (a *Attach) MarshalJSONObject(e *gojay.Encoder) {
	e.StringKey("name", a.Name)
	e.StringKeyOmitEmpty("encoding", a.Encoding)

	// attach content handling
	if a.Content != nil {
		e.AppendByte(',')                                 // begin new field
		e.AppendBytes([]byte(`"content"`))                // appends "content" key
		e.AppendByte(':')                                 // delimiter between key and value
		e.AppendByte('"')                                 // string begins
		_, err := io.Copy((*encoderWriter)(e), a.Content) // big string (unsafe, no escaping)
		if err != nil {
			panic(err) // caught by caller
		}
		e.AppendByte('"') // string ends
	}
}

My changes allows to shorten it to this:

func (a *Attach) MarshalJSONObject(e *gojay.Encoder) {
	e.StringKey("name", a.Name)
	e.StringKeyOmitEmpty("encoding", a.Encoding)
        e.ReaderToBase64Key("content", a.Content, base64.StdEncoding)
}

This PR adds similar methods for decoding (may be useful i.e. when you get response where file goes as base64-encoded string and this file should be uploaded to somewhere).

Side changes:

• decoder: better buffer reusability when parsing escape sequences
• encoder: flush buffer when it's size reaches a limit provided in SetBufFlushThreshold if target writer provided (to reduce memory consumption). Should close Periodically flush the buf to the Writer. #78