About adding rkyv support

[haruki-nikaidou]

I found that there is no issue, pull requests or discussions about the rkyv support. Can I add rkyv serialize/deserialize support with pull request?

It will expose extra ArchivedTypes if rkyv feature would be enabled, which can be considered as a breaking change. And there is a breaking change in rkyv 0.8 version that makes it unclear to implement rkyv deserialize with 0.7.x version or 0.8.x version.

[jhpratt]

A quick look at rkyv shows that they essentially want to expose the internal representation, which is unacceptable here. I reserve the right to change the internal representation of any type, and in fact have done so on a number of occasions.