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/customParcelerScoping.kt

@@ -36,7 +36,7 @@ fun box() = parcelTest { parcel ->
     parcel.unmarshall(bytes, 0, bytes.size)
     parcel.setDataPosition(0)
 
-    val test2 = readFromParcel<Ints>(parcel)
+    val test2 = parcelableCreator<Ints>().createFromParcel(parcel)
 
     assert(test2.a == -test.a)
     assert(test2.b == -test.b)