Add rkyv feature for zero-copy deserialization

[erictapen]

Hi, thanks for this great crate.

I was dissatisfied with the deserialization performance of the deser feature (using serde and bincode) and looked into zero-copy alternatives. Using rkyv I managed to read my Arena with ~17m nodes from disk in 3s (compared to 3min 30s before with serde).

This is a draft because I had to make a few types public, for it to compile. I would try to resolve that if you are generally interested in such a feature?

[saschagrunert]

@erictapen sounds good to me, it will be jus an optional feature.

[codecov]

Codecov Report

Merging #103 (80734a9) into main (3576901) will increase coverage by 5.0%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #103     +/-   ##
=======================================
+ Coverage   60.9%   66.0%   +5.0%     
=======================================
  Files          8       8             
  Lines        474     474             
  Branches     170     145     -25     
=======================================
+ Hits         289     313     +24     
  Misses        42      42             
+ Partials     143     119     -24     

[erictapen]

Blocked by rkyv/rkyv#275 for now