CHAR: adding support for CHAR(N)

Support for CHAR is implemented as a StringVal in the backend.

TODO:
  1. Parquet Reader/writer
  2. Codegen slot ref
  3. Codegen text reader
  4. Don't inline large chars
  5. update impala-hs2-server.cc with CHAR support

Change-Id: Ibba2c89cea971cb740001ea7975bf3e929150471
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4075
Reviewed-by: Nong Li <nong@cloudera.com>
Reviewed-by: Victor Bittorf <victor.bittorf@cloudera.com>
Tested-by: jenkins