Similar to `CreateArray`, we can add `CreateStruct` to create nested columns. marmbrus

Author: Xiangrui Meng <meng@databricks.com>

Closes #5195 from mengxr/SPARK-6542 and squashes the following commits:

3795c57 [Xiangrui Meng] update error message
ae7ac3e [Xiangrui Meng] move unit test to a separate suite
85dd559 [Xiangrui Meng] use NamedExpr
c78e31a [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-6542
85f3106 [Xiangrui Meng] add CreateStruct