CancellingContinuation

[swhitty]

Adds CancellingContinuation which is a thing wrapper on CheckedContinuation that detects cancellation and throws CancellationError.

Actors can then use defer { } to ensure any cleanup occurs on the actor without escaping isolated methods:
https://bugs.swift.org/browse/SR-15745

actor Foo {

  private var waiting = Set<CancellingContinuation<Void, Never>>

  func bar() async throws {
     let continuation = CancellingContinuation<Void, Never>()
     waiting.insert(continuation)
     defer { waiting.remove(continuation) }
     return try await continuation.value
  }
}

[codecov]

Codecov Report

Merging #37 (37f5be7) into main (d4647bf) will decrease coverage by 0.20%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
- Coverage   99.62%   99.42%   -0.21%     
==========================================
  Files          43       43              
  Lines        2124     2081      -43     
==========================================
- Hits         2116     2069      -47     
- Misses          8       12       +4     

Impacted Files	Coverage Δ
FlyingFox/Sources/URLSession+Async.swift	93.33% <93.75%> (-6.67%)	⬇️
FlyingSocks/Sources/CancellingContinuation.swift	96.00% <96.00%> (ø)
FlyingFox/Sources/HTTPServer+Listening.swift	100.00% <100.00%> (ø)
FlyingSocks/Sources/PollingSocketPool.swift	100.00% <100.00%> (ø)

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