Author:水如烟
SecureString类是自动加密，可锁定，可从内存删除的文本类。
基本用法如下：
Dim s As New System.Security.SecureString
s.AppendChar("a"c)
s.AppendChar("b"c)
s.AppendChar("c"c)
s.AppendChar("d"c)
s.MakeReadOnly()
Dim h As IntPtr = System.Runtime.InteropServices.Marshal.SecureStringToBSTR(s)
Dim myString As String = System.Runtime.InteropServices.Marshal.PtrToStringAuto(h, s.Length)
System.Runtime.InteropServices.Marshal.ZeroFreeBSTR(h)
s.Dispose()
