Support multiple recipient COSE operations

[falko17]

This rewrites the existing ciphers to work without requiring a self reference (instead taking in a key), and adds support for CoseSign, CoseEncrypt, and CoseMac rather than just the single-recipient variants. For this purpose, token methods with the suffix _multiple have also been implemented. Tests and documentation have been written for all new traits, macros, functions, and methods.

[coveralls]

Pull Request Test Coverage Report for Build 3981217710

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 276 of 309 (89.32%) changed or added relevant lines in 3 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 90.455%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/token/mod.rs	148	151	98.01%
src/token/tests.rs	122	128	95.31%
src/error/mod.rs	6	30	20.0%

Files with Coverage Reduction	New Missed Lines	%
src/token/mod.rs	1	97.58%
src/token/tests.rs	3	94.16%

Totals
Change from base Build 3066317205:	0.1%
Covered Lines:	1630
Relevant Lines:	1802

---

💛 - Coveralls

[pulsastrix]

Some initial comments for the parts that I got to read up until now (will do the rest as well as a deeper pass over the entire PR tomorrow).

I assume that there won't be any large comments for the remainder either, as we have already discussed the general idea behind the cipher implementations in person.

[pulsastrix]

Here's the second part of the review. As already predicted, I did not find any glaring issues that would make larger changes necessary.
If there are any remaining larger issues, I'd assume that we'd find them when we add actual implementations.

One suggestion regarding the release process for 0.4.0: In order to ensure that we don't ship a release that isn't actually functional, I'd hold off on releasing until we have at least one implementation that isn't FakeCrypto and actually links to an actual crypto library.
(I'll try to make time to write a proof-of-concept for either OpenSSL or RustCrypto this week)

[JKRhb]

(See a few very minor comments/suggestions below.)

[falko17]

@JKRhb Thank you for the review! All points should now have been resolved.

[pulsastrix]

Just one minor remark. Aside from that, there's not really much left to comment on ^^

LGTM.