[SPARK-9738] [SQL] remove FromUnsafe and add its codegen version to GenerateSafe

In https://github.com/apache/spark/pull/7752 we added `FromUnsafe` to convert nexted unsafe data like array/map/struct to safe versions. It's a quick solution and we already have `GenerateSafe` to do the conversion which is codegened. So we should remove `FromUnsafe` and implement its codegen version in `GenerateSafe`.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #8029 from cloud-fan/from-unsafe and squashes the following commits:

ed40d8f [Wenchen Fan] add the copy back
a93fd4b [Wenchen Fan] cogengen FromUnsafe