[SPARK-9448][SQL] GenerateUnsafeProjection should not share expressions across instances.

We accidentally moved the list of expressions from the generated code instance to the class wrapper, and as a result, different threads are sharing the same set of expressions, which cause problems for expressions with mutable state.

This pull request fixed that problem, and also added unit tests for all codegen classes, except GeneratedOrdering (which will never need any expressions since sort now only accepts bound references.

Author: Reynold Xin <rxin@databricks.com>

Closes #7759 from rxin/SPARK-9448 and squashes the following commits:

c09b50f [Reynold Xin] [SPARK-9448][SQL] GenerateUnsafeProjection should not share expressions across instances.