feat: Make rs-wnfs work in multithreaded contexts

[matheus23]

The main goal of this PR is to enable using rs-wnfs in multithreaded contexts, e.g. in axum webservers.
We have a test repo to check whether that works in an MVP: https://github.com/fabricedesre/wnfs-mtload/

Previously that wasn't possible, since the async futures from rs-wnfs weren't Send, so you couldn't have a multi-threaded work-stealing async runtime work on them.
The reasons they weren't sync were:

• Futures would capture an impl BlockStore, and it's not necessarily known to be Send
• Futures would capture an impl PrivateForest with the same problem
• Some functions would produce LocalBoxFuture or LocalBoxStream, which aren't Send
• We'd use async_trait(?Send) and async_recursion(?Send) which opt-in to not having Send bounds, since that's what we need for wasm
• Futures would capture internal WNFS data structures like PrivateNode, which would use Rc internally instead of Arc, see also Refactor: Consider switching from Rc to Arc #250

Some of this work was already addressed in #366. This PR should cover the rest.

---

There's a complication with Wasm, where we're e.g. using an external type extern "C" { type BlockStore; ... }, which isn't Send or Sync, and as such can't ever implement a trait BlockStore: Send + Sync.
To fix this, we're conditionally compiling in Send and Sync bounds (and Arc and Rc and similar) based on the target (See send_sync_poly.rs). This is pretty much just copying what noosphere is doing: https://github.com/subconsciousnetwork/noosphere/blob/main/rust/noosphere-common/src/sync.rs

I'm hoping eventually we just fix this and thus enable multi-threaded Wasm, too. But for now this works.

[codecov]

Codecov Report

Merging #372 (cfe2ed9) into main (30ed01b) will increase coverage by 0.19%.
The diff coverage is 57.74%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #372      +/-   ##
==========================================
+ Coverage   56.70%   56.89%   +0.19%     
==========================================
  Files          45       45              
  Lines        3333     3334       +1     
  Branches      832      830       -2     
==========================================
+ Hits         1890     1897       +7     
+ Misses        886      884       -2     
+ Partials      557      553       -4     

Files	Coverage Δ
wnfs-common/src/async_serialize.rs	33.33% <ø> (ø)
wnfs-common/src/encoding.rs	62.50% <ø> (ø)
wnfs-common/src/link.rs	19.76% <ø> (ø)
wnfs-common/src/utils/common.rs	60.00% <ø> (ø)
wnfs-hamt/src/hamt.rs	41.02% <ø> (ø)
wnfs-hamt/src/pointer.rs	21.87% <ø> (ø)
wnfs/src/lib.rs	90.00% <ø> (ø)
wnfs/src/private/file.rs	73.68% <ø> (ø)
wnfs/src/private/forest/traits.rs	100.00% <ø> (ø)
wnfs/src/private/keys/exchange.rs	69.56% <ø> (ø)
... and 24 more

[matheus23]

Replaces #368

[matheus23]

TODO:

• Look at remaining async_trait(?Send):
  • #[async_trait(?Send)] pub trait ExchangeKey
  • #[async_trait(?Send)] pub trait PrivateKey
  • WnfsBlockStore and similar wrappers in Wasm code
• Fix wnfs-wasm crate type errors.

[fabricedesre]

That looks great! Do you plan to do a version bump and release in another PR?

[matheus23]

That looks great! Do you plan to do a version bump and release in another PR?

Yep. Soon-ish ✌️