asn1: Add support for PrintableString

[facutuesca]

This adds support for encoding/decoding PrintableString.

here's an example of how it would be used:

@asn1.sequence
class MySeq:
    field: asn1.PrintableString

object = MySeq(field=asn1.PrintableString("A string"))
encoded = encode_der(object)

decoded = decode_der(MySeq, encoded)
assert decoded.field.as_str() == "A string"

A couple of implementation notes:

• In this comment, I mentioned that the idea was to use typing.NewType to define the PrintableString type. However, I had forgotten that a NewType cannot be told apart from the original type during runtime:

  Note that these checks are enforced only by the static type checker. At runtime, the statement Derived = NewType('Derived', Base) will make Derived a callable that immediately returns whatever parameter you pass it.
  src

  The actual solution was to use a Rust type PrintableString and expose it directly to Python. This type wraps a normal Python string, and can only be used through a restricted API (similar to rust-asn1's):

class PrintableString:
    def __new__(cls, inner: str) -> PrintableString: ...
    def as_str(self) -> str: ...

• The encoding errors coming from rust-asn1 are still limited to AllocationErrors without custom error messages. This is blocked by Allow custom write errors alex/rust-asn1#567, so I'll work on that next.

Part of #12283

[alex]

We shouldn't need a pybacked string, val.get().inner.bind(py) should be all we need.

[facutuesca]

fixed

[alex]

I think we want PrintableString("MyString") (or single quotes) -- using the repr of the str

[facutuesca]

fixed

[alex]

Oh, we should implement __eq__ on asn1.PrintableString.

[facutuesca]

fixed

[alex]

We should validate that inner is a valid PrintableString, rather than defering that to serialization

[facutuesca]

Should I replicate the logic from rust-asn1's PrintableString::verify ? We could also make verify public.

[alex]

asn1::PrintableString::new(s).is_none() should be enough to check if its invalid

[facutuesca]

Or call rust-asn1's PrintableString::new and discard the result

[facutuesca]

fixed