Add array literal expressibility for ToManyRelationship

Sources/JSONAPI/Resource/Relationship.swift

@@ -40,6 +40,10 @@ public struct ToManyRelationship<Relatable: JSONAPI.Relatable>: RelationshipType
 
 	public let ids: [Relatable.Identifier]
 
+	public init(ids: [Relatable.Identifier]) {
+		self.ids = ids
+	}
+
 	public init<T: JSONAPI.Relatable>(relationships: [ToOneRelationship<T>]) where T.WrappedIdentifier == Relatable.Identifier {
 		ids = relationships.map { $0.id }
 	}

Sources/JSONAPITestLib/Relationship+Literal.swift

@@ -12,3 +12,11 @@ extension ToOneRelationship: ExpressibleByNilLiteral where Relatable.WrappedIden
 		self.init(id: Relatable.WrappedIdentifier(nilLiteral: ()))
 	}
 }
+
+extension ToManyRelationship: ExpressibleByArrayLiteral {
+	public typealias ArrayLiteralElement = Relatable.Identifier
+
+	public init(arrayLiteral elements: ArrayLiteralElement...) {
+		self.init(ids: elements)
+	}
+}

Tests/JSONAPITests/TestLib/Relationship+LiteralTests.swift

@@ -14,6 +14,10 @@ class Relationship_LiteralTests: XCTestCase {
 	func test_NilLiteral() {
 		XCTAssertEqual(ToOneRelationship<TestEntity?>(id: nil), nil)
 	}
+
+	func test_ArrayLiteral() {
+		XCTAssertEqual(ToManyRelationship<TestEntity>(ids: ["1", "2", "3"]), ["1", "2", "3"])
+	}
 }
 
 // MARK: - Test types