[Based on #26] Support reference types (and a pinch of DSTs)

[HadrienG2]

This PR implements Abomonation for most references types, and a few DSTs along the way. It is what I had in mind while opening #27 . The main advantage of implementing Abomonation for these types is that it becomes much easier to serialize types containing Rust references (which are, after all, almost like every other pointer).

This PR raises a few questions that we may want to discuss here:

• Do we want to do this?
  • See #27 for a discussion of pros and cons
• Do we also want to fully support DSTs in Abomonation?
  • Currently, the Abomonation DST impls of this PR are only here 1/as a fun experiment and 2/because they are a more convenient place for deduplicating impls than free functions)
  • If we want full DST support in Abomonation, then we'll also need to make encode/decode/measure and tests accept T: ?Sized, which from a quick attempt won't be so easy. We may also want ?Sized bounds on some generic Abomonation impls like that of Box<T>
  • This would answer #18 since DSTs may currently only be passed to functions by reference.
• Can we and do we want to also implement Abomonation for dyn Trait and &[mut] dyn Trait where Trait: Abomonation?

For convenience reasons, this PR is based on #22, #24, #25 and #26. Reviewing these PRs first is recommended, but if you want to review this one in isolation, please consider doing so commit-by-commit in order to get a clean diff.

Fixes #27 .

[HadrienG2]

I have pushed a draft of the Abomonation<'bytes> memory safety fix that we discussed in #27 , but it has a bunch of FIXMEs that I hope to improve upon before merging.

[HadrienG2]

Alright, I think this one is now ready to go (though by virtue of being on the top of the PR stack, it will probably go in last).