der: add GeneraString (or more) tag(s)

[zkonge]

I also work with kerberos types.

Just like #1385 (comment) mentioned, the Tag::GeneralString is required for convenience.

It is mentioned in the https://www.oss.com/asn1/resources/asn1-made-simple/asn1-quick-reference/generalstring.html

GeneralString is too general to be implemented, therefore it is not recommended.

So an independent GeneralString type is not needed, but tags in enum is still valuable.

Going a step further, we should implement more tags in der::tag::Tag, then the AnyRef could parse more types?

[tarcieri]

Adding Tag::GeneralString is fine. Kerberos is a good use case.

Going a step further, we should implement more tags in der::tag::Tag, then the AnyRef could parse more types?

I would prefer to keep additions motivated by concrete use cases, rather than adding complexity for "completeness".

[zkonge]

GeneralString is enough. Closed by #1512.