Worker: support rkyv in additon to bincode

[droogmic]

I ran into issue bincode-org/bincode#245 in bincode, which will seemingly not be fixed (serde-rs/serde#1346) without the implementation of a new stricter flatten attribute in serde.

Although I could remove #[serde(flatten)] from my type, but that only sidesteps the problem.

It would be best if gloo_worker supports something other than serde+bincode, so this attempts to add rkyv.

[droogmic]

Help needed

I don't think I know rust well enough to know how to do this without advice.

What I tried to do is add rkyv as a feature, which then enables different trait bounds through cfg. Part way through the implementation, particularly when adding the SerDe helper trait, I began to wonder if macros weren't a better way to switch between trait bounds.

In it's current state, it doesn't compile. I don't know how to add the rkyv::Deserialize trait bound in with the correct generic type parameters.

https://doc.rust-lang.org/cargo/reference/features.html#mutually-exclusive-features
The documentation hints that mutually exclusive features may best be done using different crates or runtime switching, so I am wondering if there is a better design where the current crate API needs a breaking change to do this properly?

[droogmic]

There is also #200, which will significantly conflict with this effort.