Parcelize: Add an intrinsic to access Parcelable CREATOR fields

Fixes KT-19853.

The CREATOR field is a static field on a Parcelable class which is not
visible from Kotlin since the necessary metadata would have to be in a
Companion object which may not exist.

This commit adds a [parcelableCreator] function to kotlinx.parcelize,
which is optimized to a direct field access whenever possible.

plugins/parcelize/parcelize-compiler/testData/box/kt19853.kt

@@ -0,0 +1,26 @@
+// WITH_RUNTIME
+
+@file:JvmName("TestKt")
+package test
+
+import kotlinx.parcelize.*
+import android.os.Parcel
+import android.os.Parcelable
+
+@Parcelize
+class A(val value: Int) : Parcelable
+
+fun box() = parcelTest { parcel ->
+    parcel.writeTypedList(listOf(A(0), A(1)))
+
+    val bytes = parcel.marshall()
+    parcel.unmarshall(bytes, 0, bytes.size)
+    parcel.setDataPosition(0)
+
+    val creator = parcelableCreator<A>()
+    val deserialized = mutableListOf<A>()
+    parcel.readTypedList(deserialized, creator)
+    assert(deserialized.size == 2)
+    assert(deserialized[0].value == 0)
+    assert(deserialized[1].value == 1)
+}