Support PHP 8 and add non-blocking I/O support on Windows with PHP 8

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This changeset adds PHP 8 support and support for socket pair descriptors.

The socket pair descriptors are supported as of PHP 8 and provide native, non-blocking I/O support on Windows without any fancy workarounds. The PHP documentation doesn't currently reflect this features, but it's available in the upgrade guide here: https://www.php.net/manual/en/migration80.new-features.php

This changeset includes relevant tests, documentation and examples. In a follow-up PR I'll look into making socket pair descriptors the default for Windows, to make sure this works out-of-the-box on any platform. Explicit configuration will always remain available.

Builds on top of #84, #82, #67 and #65

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Updated to work around legacy HHVM which doesn't report pipe open mode. All tests pass now, so this is ready for review