Adding optional rkyv support.

#### Feature

Adding the [rkyv](https://github.com/rkyv/rkyv) support behind a cargo feature.

#### Benefit

Allows zero copy serialization, which is not supported by serde formats. I particularly find it very useful in my compiler.

#### Implementation

Traits can be easily derived with proc macros, for all types that derive serde Traits.

I have some experience after integrating it into my project, and it is in my interest to be supported, so I can also implement this.

#### Alternatives

I don't have notion of other libraries with same capabilities.