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/boxLib.kt

@@ -11,8 +11,3 @@ fun parcelTest(block: (Parcel) -> Unit): String {
         parcel.recycle()
     }
 }
-
-inline fun <reified T> readFromParcel(parcel: Parcel): T {
-    val creator = T::class.java.getDeclaredField("CREATOR").get(null)
-    return creator::class.java.getDeclaredMethod("createFromParcel", Parcel::class.java).invoke(creator, parcel) as T
-}