[SPARK-8935] [SQL] Implement code generation for all casts

JIRA: https://issues.apache.org/jira/browse/SPARK-8935

Author: Yijie Shen <henry.yijieshen@gmail.com>

Closes #7365 from yjshen/cast_codegen and squashes the following commits:

ef6e8b5 [Yijie Shen] getColumn and setColumn in struct cast, autounboxing in array and map
eaece18 [Yijie Shen] remove null case in cast code gen
fd7eba4 [Yijie Shen] resolve comments
80378a5 [Yijie Shen] the missing self cast
611d66e [Yijie Shen] Bug fix: NullType & primitive object unboxing
6d5c0fe [Yijie Shen] rebase and add Interval codegen
9424b65 [Yijie Shen] tiny style fix
4a1c801 [Yijie Shen] remove CodeHolder class, use function instead.
3f5df88 [Yijie Shen] CodeHolder for complex dataTypes
c286f13 [Yijie Shen] moved all the cast code into class body
4edfd76 [Yijie Shen] [WIP] finished primitive part