Implement futures

[dotcarmen]

This PR implements wgpuInstanceWaitAny and modifies already-implemented functions that rely on this functionality.

Apologies for the large diff. It's mostly contained to the first commit, which refactors Context to be a struct that contains wgc::global::Global instead of being an alias to that type.

Additionally, this PR somewhat replicates logic that's already implemented in wgc, but are hidden from the public api (aka declared using pub(crate) and are not re-exported elsewhere). Unfortunately, this includes wgc::lock, which I didn't duplicate here - instead, I simply rely on parking_lot without incorporating the feature-gated implementations from wgc.

[dotcarmen]

i won't keep force-pushing now that it's open for review - was trying to keep the set of commits small for maintainers' convenience :) i recommend looking at the 2nd and 3rd commits to isolate away the changes from the refactor

[dotcarmen]

i can reduce the diff with impl Deref, let me know if this is preferred :)

[dotcarmen]

i figured it's simple to keep this as a hot loop than to park the thread, but i can also replace the whole loop with:

let mut success = false;
{
  let registry = context.futures.read();
  for future in futures.iter_mut() {
    // ... the same
  }
}

if success {
  return native::WGPUWaitStatus_Success;
}

std::thread::sleep(timeout_ns);
let registry = context.futures.read();
for future in futures.iter_mut() {
  // ... the same
}

return if success { native::WGPUWaitStatus_Success } else { native::WGPUWaitStatus_TimedOut };

[dotcarmen]

eg

let mut reg = FutureRegistry::default();
for _ in 0..253 {
  reg.completed_future();
}

// still 0 allocations, `reg.futures.len() == 0`
reg.completed_future();
// 1 allocation, `reg.futures.len() == 255`
reg.incomplete_future();

[dotcarmen]

wait no, reg.futures.len() == 1 in the 2nd scenario since the underlying IdentityManager is able to only increment the epoch 😅

[dotcarmen]

making this a draft again - i need to make sure to call poll functions appropriately