Adopt ~Copyable in Subprocess

[iCharlesHu]

This PR introduces ~Copyable into Subprocess, including CreatedPipe, TrackedFileDescriptor, and TrackedDispatchIO.

Resolves #37
Resolves #2

Additionally, we opted to slightly modify the API by moving the .standardOutput and .standardError AsyncSequence from Execution to the closure parameter, alongside Execution. This adjustment eliminated the necessity for Atomic (and AtomicBox) within Execution.

Furthermore, this change rendered SequenceOutput internal, as we now rely on overloads to infer the output and error types.

[jakepetroules]

Instead of using a task group, how about using async let? It's shorter, simpler, and takes better advantage of the type system (no force unwraps or OutputCapturingState helper type needed):

...
    ) { execution, inputIO, outputIO, errorIO in
        // Write input, capture output and error in parallel
        var inputIOBox: TrackedPlatformDiskIO? = consume inputIO
        var outputIOBox: TrackedPlatformDiskIO? = consume outputIO
        var errorIOBox: TrackedPlatformDiskIO? = consume errorIO

        var outputIOContainer: TrackedPlatformDiskIO? = outputIOBox.take()
        var errorIOContainer: TrackedPlatformDiskIO? = errorIOBox.take()
        async let stdout = output.captureOutput(
            from: outputIOContainer.take()
        )
        async let stderror = error.captureOutput(
            from: errorIOContainer.take()
        )

        // Write span at the same isolation
        if let writeFd = inputIOBox.take() {
            let writer = StandardInputWriter(diskIO: writeFd)
            _ = try await writer.write(input._bytes)
            try await writer.finish()
        }

        return try await (
            processIdentifier: execution.processIdentifier,
            standardOutput: stdout,
            standardError: stderror
        )
    }
...

[jakepetroules]

Should this be fatalError? The string of preconditionFailure is not retained in release builds, which will make the cause of crashes here, harder to identify.

[jakepetroules]

Suggest using if let inputReadFileDescriptor { so you don't need the force unwrap below.