Reflection: add KTypeProjection and KType.arguments

#KT-8998 In Progress

core/builtins/src/kotlin/reflect/KType.kt

@@ -29,6 +29,12 @@ public interface KType {
      */
     public val classifier: KClassifier?
 
+    /**
+     * Type arguments passed for the parameters of the classifier in this type.
+     * For example, in the type `Array<out Number>` the only type argument is `out Number`.
+     */
+    public val arguments: List<KTypeProjection>
+
     /**
      * `true` if this type was marked nullable in the source code.
      *
@@ -42,9 +48,54 @@ public interface KType {
      *
      * ```
      * fun <T> foo(t: T) {
-     *     // isMarkedNullable == false for t's type, but t can be null here
+     *     // isMarkedNullable == false for t's type, but t can be null here when T = "Any?"
      * }
      * ```
      */
     public val isMarkedNullable: Boolean
 }
+
+/**
+ * Represents a type projection. Type projection is usually the argument to another type in a type usage.
+ * For example, in the type `Array<out Number>`, `out Number` is the covariant projection of the type represented by the class `Number`.
+ *
+ * Type projection is either the star projection, or an entity consisting of a specific type plus optional variance.
+ *
+ * See the [Kotlin language documentation](http://kotlinlang.org/docs/reference/generics.html#type-projections)
+ * for more information.
+ */
+public sealed class KTypeProjection {
+    /**
+     * The type specified in the projection, or `null` if this is a star projection.
+     */
+    public abstract val type: KType?
+
+    /**
+     * Invariant projection of a type. Invariant projection is just the type itself, without any use-site variance modifiers applied to it.
+     * For example, in the type `Set<String>`, `String` is an invariant projection of the type represented by the class `String`.
+     */
+    public data class Invariant(override val type: KType) : KTypeProjection()
+
+    /**
+     * Contravariant projection of a type, denoted by the `in` modifier applied to a type.
+     * For example, in the type `MutableList<in Number>`, `in Number` is a contravariant projection of the type of class `Number`.
+     */
+    public data class In(override val type: KType) : KTypeProjection()
+
+    /**
+     * Covariant projection of a type, denoted by the `out` modifier applied to a type.
+     * For example, in the type `Array<out Number>`, `out Number` is a covariant projection of the type of class `Number`.
+     */
+    public data class Out(override val type: KType) : KTypeProjection()
+
+    /**
+     * Star projection, denoted by the `*` character.
+     * For example, in the type `KClass<*>`, `*` is the star projection.
+     * See the [Kotlin language documentation](http://kotlinlang.org/docs/reference/generics.html#star-projections)
+     * for more information.
+     */
+    public object Star : KTypeProjection() {
+        override val type: KType?
+            get() = null
+    }
+}