IMPALA-1238: Impala supports length() but not char_length()

 Adds aliases char_length and character_length for the length()
 builtin. These are valid for ASCII.

Change-Id: I934d997f2c6d372ed12e7221efc1a574d68e01f3
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1802
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins