Rollup merge of #153283 - LukeMathWalker:add-rkyv-support, r=aDotInTheVoid

feat(rustdoc-json): Add optional support for rkyv (de)serialization

## Motivation

The JSON documents produced by `rustdoc-json` are _big_. More often than not, tools need to access a small fraction of that output—e.g. a couple of types from a transitive dependency, or a subset of the fields on a given `rustdoc-json-types` type.

Using a binary (de)serialization format and a cache helps to drive down the performance cost of deserialization: you invoke `rustdoc-json` to get the JSON output you need, re-serialize it using a more perfomant format as target (e.g. `bincode` or `postcard`) and thus amortize the cost of future queries that hit the persistent cache rather than `rustdoc-json`.
This is _better_, but still not great: the deserialization cost for crates like `std` still shows up prominently in flamegraphs.

## An Alternative Approach: rkyv

`rkyv` provides a different opportunity: you avoid paying the deserialization cost _upfront_ thanks to [zero-copy deserialization](https://rkyv.org/zero-copy-deserialization.html).
You're often able to determine if you need a certain entry from the JSON document using the archived version of that type, thus incurring the full deserialization cost only for the subset of items you actually need ([example](https://github.com/LukeMathWalker/pavex/pull/574/changes/d067e7e0cfa38da5b0a3c8713b1c53beb0b29a5d)).

## The Change

This PR adds support for `rkyv` behind a feature flag (`rkyv_0_8`).
For most types, it's a straight-forward `derive(rkyv::Archive, rkyv::Serialize, rkyv::Deserialize)` annotation. For co-recursive types, we need to adjust the generated bounds, using the techniques from [`rkyv`'s JSON example](https://github.com/rkyv/rkyv/blob/985b0230a0b9cb9fce4a4ee9facb6af148e27c8e/rkyv/examples/json_like_schema.rs).

I have added new round-trip tests to ensure `rkyv` works as expected.

r? @aDotInTheVoid