Create a IsMapped expression

This will allow us to ensure that certain classes are always in a map.
This is useful, for example, to ensure that DTOs like commands and
events are always set in a map, so that a serializer knows how
to serialize/deserialize them.

src/Expression/ForClasses/IsMapped.php

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+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Expression\ForClasses;
+
+use Arkitect\Analyzer\ClassDescription;
+use Arkitect\Expression\Description;
+use Arkitect\Expression\Expression;
+use Arkitect\Rules\Violation;
+use Arkitect\Rules\ViolationMessage;
+use Arkitect\Rules\Violations;
+
+final class IsMapped implements Expression
+{
+    public const POSITIVE_DESCRIPTION = 'should exist in the list';
+
+    /** @var array */
+    private $list;
+
+    public function __construct(array $list)
+    {
+        $this->list = array_flip($list);
+    }
+
+    public function describe(ClassDescription $theClass, string $because): Description
+    {
+        return new Description(self::POSITIVE_DESCRIPTION, $because);
+    }
+
+    public function evaluate(ClassDescription $theClass, Violations $violations, string $because): void
+    {
+        if (isset($this->list[$theClass->getFQCN()])) {
+            return;
+        }
+
+        $violation = Violation::create(
+            $theClass->getFQCN(),
+            ViolationMessage::selfExplanatory($this->describe($theClass, $because))
+        );
+
+        $violations->add($violation);
+    }
+}

tests/Unit/Expressions/ForClasses/IsMappedTest.php

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+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses;
+
+use Arkitect\Analyzer\ClassDescriptionBuilder;
+use Arkitect\Expression\ForClasses\IsMapped;
+use Arkitect\Rules\Violations;
+use PHPUnit\Framework\TestCase;
+
+final class IsMappedTest extends TestCase
+{
+    public function test_it_should_not_add_violation_if_fqcn_is_in_list(): void
+    {
+        $listedFqcn = 'App\SharedKernel\Component\MyClass';
+        $list = [
+            'a' => $listedFqcn,
+            'b' => 'App\SharedKernel\Component\MyOtherClass',
+        ];
+        $expression = new IsMapped($list);
+        $classDescription = (new ClassDescriptionBuilder())->setClassName($listedFqcn)->build();
+
+        $violations = new Violations();
+        $expression->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(0, $violations->count());
+    }
+
+    public function test_it_should_add_violation_if_fqcn_is_not_in_list(): void
+    {
+        $nonListedFqcn = 'App\SharedKernel\Component\MyClass';
+        $list = [
+            'a' => 'App\SharedKernel\Component\SomeClass',
+            'b' => 'App\SharedKernel\Component\MyOtherClass',
+        ];
+        $expression = new IsMapped($list);
+        $classDescription = (new ClassDescriptionBuilder())->setClassName($nonListedFqcn)->build();
+
+        $violations = new Violations();
+        $expression->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(1, $violations->count());
+        self::assertEquals(IsMapped::POSITIVE_DESCRIPTION, $expression->describe($classDescription, '')->toString());
+    }
+}