Add a broadcast async sequence (grpc#1684)

Motivation:

To support retries and hedging we need a way to buffer elements over
time that can support multiple consumers concurrently and allows for
consumers to start consuming after some elements have been produced.

An `AsyncSequence` fits this quite naturally but we don't yet have a
general purpose implementat that fits this requirement. This change
adds `BroadcastAsyncSequence` which isn't a general purpose async
sequence but instead is tailored to the needs of grpc for hedging and
retries. This means it supports a low number of concurrent iterators and
maintains a limited size internal buffer and drops the slowest consumers
when the buffer becomes full.

Modifications:

- Add a `BroadcastAsyncSequence` and tests
- Made a bunch of things inlinable/usableFromInline which necessitated a
  switch from `@_spi(Testing)` to `@testable` imports.
- Rename the 'Stream' directory to 'Streaming'

Result:

- `BroadcastAsyncSequence` can be used to implement retries and hedging.

Author: glbrntt

Package.swift

@@ -158,7 +158,7 @@ extension Target {
   static let grpcCore: Target = .target(
     name: "GRPCCore",
     dependencies: [
-      .dequeModule
+      .dequeModule,
     ],
     path: "Sources/GRPCCore"
   )

Sources/GRPCCore/Call/Client/ClientResponse.swift

@@ -371,8 +371,7 @@ extension ClientResponse.Stream {
 
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 extension ClientResponse.Single {
-  @_spi(Testing)
-  public init(stream response: ClientResponse.Stream<Message>) async {
+  init(stream response: ClientResponse.Stream<Message>) async {
     switch response.accepted {
     case .success(let contents):
       do {

Sources/GRPCCore/Stream/AsyncSequenceOfOne.swift renamed to Sources/GRPCCore/Streaming/Internal/AsyncSequenceOfOne.swift

@@ -17,38 +17,46 @@
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 extension RPCAsyncSequence {
   /// Returns an ``RPCAsyncSequence`` containing just the given element.
-  @_spi(Testing)
-  public static func one(_ element: Element) -> Self {
+  @inlinable
+  static func one(_ element: Element) -> Self {
     return Self(wrapping: AsyncSequenceOfOne<Element, Never>(result: .success(element)))
   }
 
   /// Returns an ``RPCAsyncSequence`` throwing the given error.
-  @_spi(Testing)
-  public static func throwing<E: Error>(_ error: E) -> Self {
+  @inlinable
+  static func throwing<E: Error>(_ error: E) -> Self {
     return Self(wrapping: AsyncSequenceOfOne<Element, E>(result: .failure(error)))
   }
 }
 
 /// An `AsyncSequence` of a single value.
+@usableFromInline
 @available(macOS 10.15, iOS 13.0, tvOS 13, watchOS 6, *)
-private struct AsyncSequenceOfOne<Element: Sendable, Failure: Error>: AsyncSequence {
-  private let result: Result<Element, Failure>
+struct AsyncSequenceOfOne<Element: Sendable, Failure: Error>: AsyncSequence {
+  @usableFromInline
+  let result: Result<Element, Failure>
 
+  @inlinable
   init(result: Result<Element, Failure>) {
     self.result = result
   }
 
+  @inlinable
   func makeAsyncIterator() -> AsyncIterator {
     AsyncIterator(result: self.result)
   }
 
+  @usableFromInline
   struct AsyncIterator: AsyncIteratorProtocol {
-    private var result: Result<Element, Failure>?
+    @usableFromInline
+    private(set) var result: Result<Element, Failure>?
 
-    fileprivate init(result: Result<Element, Failure>) {
+    @inlinable
+    init(result: Result<Element, Failure>) {
       self.result = result
     }
 
+    @inlinable
     mutating func next() async throws -> Element? {
       guard let result = self.result else { return nil }