func (q *UpdateBuilder) Set(f Field, v interface{}) *UpdateBuilder {
	q.set = append(q.set, newSet(f, v))
	return q
}
Set adds an update to the SET clause
package qb
import (
	"reflect"
	"strings"
)
type set struct {
	Field Field
	Value Field
}
func newSet(f Field, v interface{}) set {
	f1, ok := f.(*TableField)
	if !ok {
		panic(`Cannot use non-table field in update`)
	}
	if f1.ReadOnly {
		panic(`Cannot update read-only field`)
	}
	f2 := MakeField(v)
	return set{f1, f2}
}
type UpdateBuilder struct {
	t   *Table
	c   []Condition
	set []set
}