func newParser(value []byte) (p *parser) {
	p = &parser{
		r:      &libio.Reader{},
		sepKey: []byte{'='},
		sepVal: []byte{';'},
		tags:   make(map[tagKey]empty),
	}
	p.r.Init(value)
	return p
}
newParser create and initialize new parser for DKIM Signature.
package dkim
import (
	"fmt"
	"github.com/shuLhan/share/lib/ascii"
	libio "github.com/shuLhan/share/lib/io"
)
type empty struct{}
type parser struct {
	tags   map[tagKey]empty  
	sepKey []byte
	sepVal []byte
	r      *libio.Reader
	c      byte
	isTerm bool
	tok    []byte
}