Use ASN.1 DER instead of MessagePack

[fjarri]

This PR is supposed to replace MessagePack with ASN.1 DER as the preferred binary format.

There are several problems though:

• serde_asn1_der is semi-abandoned and reports the format as human-readable, causing storage inefficiencies.
• picky-asn1-der, a fork of serde_asn1_der, which is used in this PR, does not support Optional fields (or rather, it does not serialize None, so a subsequent deserialization fails - let's wait for the resolution on Support for Optional in ASN.1 Devolutions/picky-rs#201)
• Also picky-asn1-der does not support mappings, which is not critical for this crate, but nucypher-core has those in its data structures.

Since the whole point of this change is to use a "standard" format, we should establish whether the lack of support for Optional and mappings is a limitation of the standard, or just a lack of a feature in picky-asn1-der. In the latter case we can make a PR to it, adding that support. Evidently, mappings do occur in formats that use ASN.1, e.g. VarBind here encoded as a sequence of 2-tuples.

[codecov-commenter]

Codecov Report

Merging #111 (aaeb755) into master (5080d6d) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #111   +/-   ##
=======================================
  Coverage   48.44%   48.44%           
=======================================
  Files          17       17           
  Lines         997      997           
=======================================
  Hits          483      483           
  Misses        514      514           

Impacted Files	Coverage Δ
umbral-pre/src/capsule_frag.rs	79.68% <ø> (ø)
umbral-pre/src/key_frag.rs	84.94% <ø> (ø)
umbral-pre/src/serde_bytes.rs	41.86% <ø> (ø)
umbral-pre/src/traits.rs	0.00% <0.00%> (ø)

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[KizzyCode]

Just as a side note: serde_asn1_der is not really abandoned – currently I'm working on a v1 release of the underlying asn1_der-crate (which however may take some time since I don't really want to soften it's no-panic approach and subsequent "guarantees").

Since serde_asn1_der serves as a simple interface to asn1_der, it cannot really progress independently (apart from bug-fixes obviously). However, I'm always open for feature-requests or PRs, especially if I missed a useful feature that can be implemented on top of the current asn1_der crate version 😊

Since DER's type system is not fully compatible to serde's type system, there is probably still a lot of room for improvements I have missed.