Replace AsyncBackpressuredStream with updated implementation

Motivation

As a follow up to #27, we noted it would be good to align on the latest draft implementation of SE-0406 (AsyncStream with backpressure) to both pickup the latest improvements in performance and correctness, and to minimise the churn if/when this lands in the standard library or standalone package.

Modifications

In order to simplify reviewing the following modifications have been made in independent commits:

Add updated SE-0406 implementation as BufferedStream, incl. tests: Skim over this—it's vendored in wholesale.
Port the custom watermark support to BufferedStream: Skim over this—it's a 1:1 port of the logic that was added to AsyncBackpressuredStream.
Switch from AsyncBackpressuredStream to BufferedStream in delegate: Review this—it's a minimal change.
Remove AsyncBackpressuredStream and its vendored locks: Skim over this—it's removing the old implementation.

Result

No functional change, but the internal async sequence we're using should be more robust, performant, and more likely to match a future standard library type.

Test Plan

The new vendored BufferedStream actually comes with a much greater number of vendored tests than the previous revision.
Also ported the tests from this repo for the custom watermark logic.
All our URLSessionTransport-specific tests continue to pass.

lgtm. Please once this is released, double check that NIO isn't brought in during package resolution when depending on the transport as a library.

So here's an example package that depends on the source branch for this PR, showing that it doesn't clone NIO. The only new dependency this release will bring in is swift-collections.

❯ tree
.
├── Package.swift
└── Sources
    └── OpenAPIURLSessionAdopter
        └── Example.swift

❯ cat Package.swift
// swift-tools-version: 5.9
import PackageDescription

let package = Package(
    name: "swift-openapi-urlsession-adopter",
    platforms: [.macOS(.v12)],
    dependencies: [
        .package(
            url: "https://github.com/simonjbeaumont/swift-openapi-urlsession",
            branch: "sb/update-async-stream-implementation"
        ),
    ],
    targets: [
        .target(
            name: "OpenAPIURLSessionAdopter",
            dependencies: [
                .product(name: "OpenAPIURLSession", package: "swift-openapi-urlsession"),
            ]
        ),
    ]
)

❯ cat Sources/OpenAPIURLSessionAdopter/Example.swift
import OpenAPIURLSession

func f() {
    _ = OpenAPIURLSession.URLSessionTransport()
}

❯ swift build
Fetching https://github.com/simonjbeaumont/swift-openapi-urlsession from cache
Fetched https://github.com/simonjbeaumont/swift-openapi-urlsession (0.53s)
Fetching https://github.com/apple/swift-collections from cache
Fetching https://github.com/apple/swift-openapi-runtime from cache
Fetching https://github.com/apple/swift-http-types from cache
Fetched https://github.com/apple/swift-http-types (0.52s)
Fetched https://github.com/apple/swift-collections (0.54s)
Fetched https://github.com/apple/swift-openapi-runtime (0.65s)
Computing version for https://github.com/apple/swift-collections
Computed https://github.com/apple/swift-collections at 1.0.5 (0.50s)
Computing version for https://github.com/apple/swift-openapi-runtime
Computed https://github.com/apple/swift-openapi-runtime at 0.3.6 (0.43s)
Computing version for https://github.com/apple/swift-http-types
Computed https://github.com/apple/swift-http-types at 1.0.0 (0.52s)
Creating working copy for https://github.com/simonjbeaumont/swift-openapi-urlsession
Working copy of https://github.com/simonjbeaumont/swift-openapi-urlsession resolved at sb/update-async-stream-implementation
Creating working copy for https://github.com/apple/swift-openapi-runtime
Working copy of https://github.com/apple/swift-openapi-runtime resolved at 0.3.6
Creating working copy for https://github.com/apple/swift-collections
Working copy of https://github.com/apple/swift-collections resolved at 1.0.5
Creating working copy for https://github.com/apple/swift-http-types
Working copy of https://github.com/apple/swift-http-types resolved at 1.0.0
Building for debugging...
[84/84] Compiling OpenAPIURLSessionAdopter Example.swift
Build complete! (17.54s)

❯ cat Package.resolved | jq .pins[].location
"https://github.com/apple/swift-collections"
"https://github.com/apple/swift-http-types"
"https://github.com/apple/swift-openapi-runtime"
"https://github.com/simonjbeaumont/swift-openapi-urlsession"

❯ swift package show-dependencies
.
└── swift-openapi-urlsession<https://github.com/simonjbeaumont/swift-openapi-urlsession@unspecified>
    ├── swift-openapi-runtime<https://github.com/apple/swift-openapi-runtime@0.3.6>
    │   └── swift-http-types<https://github.com/apple/swift-http-types@1.0.0>
    └── swift-collections<https://github.com/apple/swift-collections@1.0.5>

apple / swift-openapi-urlsession