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/kt41553_2.kt

@@ -21,9 +21,10 @@ fun box() = parcelTest { parcel ->
     parcel.unmarshall(bytes, 0, bytes.size)
     parcel.setDataPosition(0)
 
-    val a2 = readFromParcel<A>(parcel)
+    val parcelableCreator = parcelableCreator<A>()
+    val a2 = parcelableCreator.createFromParcel(parcel)
     assert(a2.params != null)
     assert(Arrays.equals(a1.params!!, a2.params!!))
-    val b2 = readFromParcel<A>(parcel)
+    val b2 = parcelableCreator.createFromParcel(parcel)
     assert(b1 == b2)
 }