der: unified generic *String/*StringRef types

[tarcieri]

We currently use the impl_string_type! macro to write the bodies of various ASN.1 string types.

For each ASN.1 type, we support an owned (*String) and borrowed (*StringRef) type.

Instead I think we could potentially have two generic types, e.g. StringType and StringRefType.

We need to make them generic around a value (the tag) though, so this seems like a use case for const generics. Unfortunately we model Tag as an enum, which we can't use as a const generic parameter until the adt_const_params feature is stabilized. However, this enum is repr(u8), so it's possible to use e.g. const TAG: u8 as the const generic parameter, casting/converting the Tag into a u8 (temporarily until adt_const_params are stabilized).

Really the various string types differ in two respects:

1. The tag, which is captured as a const generic parameter per above
2. Validation rules, i.e. what characters are allowed in the constructor

To capture these concerns I propose making a trait which can be impl'd for a const generic StringType/StringTypeRef containing these validation rules.

Then the existing types can become type aliases, e.g. pub type Ia5String = StringType<Tag::Ia5String as u8>