der: Error stacktraces

https://github.com/RustCrypto/formats/blob/bd840638835fcf66c4ffc3baed6d7c3b727c4ef3/der/src/decode.rs#L26-L28

I think associated Error would be useful inside Reader trait. [^1]

Motivation

To prove usefulness, suppose we want to return annotated errors with .field stacktrace.

Example of such stacktrace from invalid PKCS#15:

Error: from_der_type CIOChoice

Caused by:
   0: .choice AuthObjects
   0: .choice Objects
   0: [0]
   0: (AuthenticationObjectChoice)
   0: .choice Pwd
   0: .type_attributes
   0: .pwd_reference

   1: incorrect length for INTEGER at DER byte 57

Because some ASN.1 DER definitions are really complicated, such error stacktrace is very helpful.
Field names in returned error are possible, because derive Sequence may generate .wrap_err_field("fieldname") calls.

Why not use Decode::Error?

Such errors may originate from x509-cert, as it is a part of PKCS#15:

CertificateChoice ::= CHOICE {
    x509Certificate CertificateObject {X509CertificateAttributes},
    -- ...
    spkiCertificate [1] CertificateObject {SPKICertificateAttributes},

Moreover, existing crates using der will benefit from such Decode error stacktrace feature, similar to my clarify Encode feature.

[^1]: I mentioned that some time ago https://github.com/RustCrypto/formats/issues/1053#issuecomment-2393628486

Read der/src/decode.rs at lines 26-28 and the linked discussion in issue #1053. Examine how the Reader trait currently handles errors and how an associated Error could support the annotated field stacktrace described here. Done means the proposed error design is implemented and supports the PKCS#15-style trace.