func (structDescriptor *StructDescriptor) GetField(fieldName string) *Binding {
	for _, binding := range structDescriptor.Fields {
		if binding.Field.Name() == fieldName {
			return binding
		}
	}
	return nil
}
GetField get one field from the descriptor by its name.
Can not use map here to keep field orders.
package jsoniter
import (
	"fmt"
	"github.com/modern-go/reflect2"
	"reflect"
	"sort"
	"strings"
	"unicode"
	"unsafe"
)
var typeDecoders = map[string]ValDecoder{}
var fieldDecoders = map[string]ValDecoder{}
var typeEncoders = map[string]ValEncoder{}
var fieldEncoders = map[string]ValEncoder{}
var extensions = []Extension{}
type StructDescriptor struct {
	Type   reflect2.Type
	Fields []*Binding
}