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/newArrayParceler.kt

@@ -35,7 +35,7 @@ fun box() = parcelTest { parcel ->
     parcel.unmarshall(bytes, 0, bytes.size)
     parcel.setDataPosition(0)
 
-    val creator = User::class.java.getDeclaredField("CREATOR").get(null) as Parcelable.Creator<User>
+    val creator = parcelableCreator<User>()
     val result = parcel.createTypedArray(creator)
 
     assert(result.size == 3)