Add a Pipe type.

[grynspan]

This PR adds a move-only wrapper for POSIX and Windows anonymous pipes (i.e. those created with pipe(2).) We will need pipes in order to implement platform-independent interprocess communication such as that required by exit tests or future adoption of distributed actors.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test Windows

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test Linux

[grynspan]

@swift-ci test macOS

[kateinoigakukun]

Unfortunately, WASI doesn't have pipe...

/home/build-user/swift-testing/Sources/Testing/Support/FileHandle.swift:449:20: error: cannot find 'pipe' in scope
447 |         }
448 | #else
449 |         guard 0 == pipe(fds.baseAddress) else {
    |                    `- error: cannot find 'pipe' in scope
450 |           throw CError(rawValue: swt_errno())
451 |         }

[grynspan]

We can add a SWT_NO_PIPE flag for this. I'll open a PR in the morning (my time.) Would you mind filing an issue?

[kateinoigakukun]

@grynspan Thanks! Opened an issue here 🙏 #698