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/parcelizeCustomValueClass.kt

@@ -33,6 +33,6 @@ fun box() = parcelTest { parcel ->
     parcel.unmarshall(bytes, 0, bytes.size)
     parcel.setDataPosition(0)
 
-    val data2 = readFromParcel<Data>(parcel)
+    val data2 = parcelableCreator<Data>().createFromParcel(parcel)
     assert(data2.uuid.uuid == data.uuid.uuid)
 }