[SPARK-9157] [SQL] codegen substring

https://issues.apache.org/jira/browse/SPARK-9157

Author: Tarek Auel <tarek.auel@googlemail.com>

Closes #7534 from tarekauel/SPARK-9157 and squashes the following commits:

e65e3e9 [Tarek Auel] [SPARK-9157] indent fix
44e89f8 [Tarek Auel] [SPARK-9157] use EMPTY_UTF8
37d54c4 [Tarek Auel] Merge branch 'master' into SPARK-9157
60732ea [Tarek Auel] [SPARK-9157] created substringSQL in UTF8String
18c3576 [Tarek Auel] [SPARK-9157][SQL] remove slice pos
1a2e611 [Tarek Auel] [SPARK-9157][SQL] codegen substring