func (rb *Bitmap) FromBase64(str string) (int64, error) {
	data, err := base64.StdEncoding.DecodeString(str)
	if err != nil {
		return 0, err
	}
	buf := bytes.NewBuffer(data)
	return rb.ReadFrom(buf)
}
FromBase64 deserializes a bitmap from Base64
package roaring
import (
	"bufio"
	"bytes"
	"encoding/base64"
	"fmt"
	"io"
	"strconv"
)
type Bitmap struct {
	highlowcontainer roaringArray
}
func (rb *Bitmap) ToBase64() (string, error) {
	buf := new(bytes.Buffer)
	_, err := rb.WriteTo(buf)
	return base64.StdEncoding.EncodeToString(buf.Bytes()), err
}