提议：smx509 包用 type alias 来减少与标准库的代码差异

[xuyang2]

如：

type PEMCipher = x509.PEMCipher

const (
	PEMCipherDES    = x509.PEMCipherDES
	PEMCipher3DES   = x509.PEMCipher3DES
	PEMCipherAES128 = x509.PEMCipherAES128
	PEMCipherAES192 = x509.PEMCipherAES192
	PEMCipherAES256 = x509.PEMCipherAES256
)

type PublicKeyAlgorithm = x509.PublicKeyAlgorithm
...

另外 Certificate 和 CertificateRequest 似乎可以改成：

type Certificate x509.Certificate
type CertificateRequest x509.CertificateRequest

来减少与标准库的代码差异。

如果可以这么做，我可以提交PR。

[emmansun]

当然可以，欢迎修改。