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ARROW-210:  Cleanup of the string related types in C++ code base

One thing that is worth discussing is if char types should also be removed (if they aren't i'll add the missing unit tests).

I also moved CharType to type.h which seems more consistent with existing code.  I can clean it up either way in a follow-up review if we decide with want to push types into their corresponding Array headers.

Author: Micah Kornfield <emkornfield@gmail.com>

Closes #85 from emkornfield/emk_string_types_wip and squashes the following commits:

4414816 [Micah Kornfield] remove CHAR from parquet
6f0634c [Micah Kornfield] remove char type and add dcheck
58bfcc9 [Micah Kornfield] fix style of char_type_
1e0152d [Micah Kornfield] wip