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/valueClassWrapper.kt

@@ -29,9 +29,9 @@ fun box() = parcelTest { parcel ->
     parcel.unmarshall(bytes, 0, bytes.size)
     parcel.setDataPosition(0)
 
-    val data2 = readFromParcel<Wrapper>(parcel)
+    val data2 = parcelableCreator<Wrapper>().createFromParcel(parcel)
     assert(data2 == data)
 
-    val none2 = readFromParcel<NullableWrapper>(parcel)
+    val none2 = parcelableCreator<NullableWrapper>().createFromParcel(parcel)
     assert(none2 == none)
 }