Swoole docs (#326)

* add Swoole backend

* phpstan: ignore swoole

* feat: swoole process scheduler

* fix(swoole): redo tasks when hash collision happens

* chore(swoole): make sure coroutines are at the root of the scheduler

* chore(swoole): set affinity / bind worker to a specific CPU core

* chore(swoole): use igbinary if available

* fix: remove comment

* fix(swoole): worker cpu affinity

* fix(swoole): cpu num

* feat: scheduler improvements

* style

* chore(swoole): remove unnecessary atomics

* chore(swoole): php backwards compatibility

* fix: phpstan, socket message size

* fix: uncomment test

* style: composer fix

* docs: Swoole backend

docs/backends/swoole.md

@@ -0,0 +1,18 @@
+<span style="float:right;"><a href="https://github.com/RubixML/ML/blob/master/src/Backends/Swoole.php">[source]</a></span>
+
+# Swoole
+[Swoole](https://swoole.com)/[OpenSwoole](https://openswoole.com/) is an async PHP extension that also supports multiprocessing.
+
+!!! tip
+    Swoole backend makes use of [Igbinary](https://www.php.net/manual/en/intro.igbinary.php) serializer. If you need to
+    optimize the memory usage (or getting out-of-memory errors) consider installing [Igbinary](https://www.php.net/manual/en/intro.igbinary.php).
+
+## Example
+
+No parameters are required. It's a drop-in replacement for the [Serial](backends/serial.md) backend.
+
+```php
+use Rubix\ML\Backends\Swoole;
+
+$backend = new Swoole();
+```