add entityBagOf and mutableEntityBagOf functions

src/commonMain/kotlin/com/github/quillraven/fleks/collection/entityBag.kt

@@ -7,6 +7,24 @@ import kotlin.math.max
 import kotlin.math.min
 import kotlin.random.Random
 
+/**
+ * Returns a new read-only [EntityBag] of given [entities].
+ */
+fun entityBagOf(vararg entities: Entity): EntityBag {
+    return MutableEntityBag(entities.size).apply {
+        entities.forEach { this += it }
+    }
+}
+
+/**
+ * Returns a new [MutableEntityBag] with the given [entities].
+ */
+fun mutableEntityBagOf(vararg entities: Entity): MutableEntityBag {
+    return MutableEntityBag(entities.size).apply {
+        entities.forEach { this += it }
+    }
+}
+
 interface EntityBag {
     /**
      * Returns the size of the [EntityBag].

src/commonTest/kotlin/com/github/quillraven/fleks/collection/EntityBagTest.kt

@@ -626,4 +626,26 @@ class EntityBagTest {
         assertEquals(bagOf(testEntity1, testEntity2), testBag.take(2))
         assertEquals(bagOf(testEntity1, testEntity2), testBag.take(3))
     }
+
+    @Test
+    fun testEntityBagOf() {
+        var bag = entityBagOf(testEntity1, testEntity2)
+        assertEquals(2, bag.size)
+        assertTrue { testEntity1 in bag }
+        assertTrue { testEntity2 in bag }
+
+        bag = entityBagOf()
+        assertEquals(0, bag.size)
+    }
+
+    @Test
+    fun testMutableEntityBagOf() {
+        var bag = mutableEntityBagOf(testEntity1, testEntity2)
+        assertEquals(2, bag.size)
+        assertTrue { testEntity1 in bag }
+        assertTrue { testEntity2 in bag }
+
+        bag = mutableEntityBagOf()
+        assertEquals(0, bag.size)
+    }
 }