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ARROW-5334: [C++] Ensure all type classes end with "Type"

This is for consistency and also for readability ("Integer" or "Number" do not explicitly stand out as *type* classes or instances).

Author: Antoine Pitrou <antoine@python.org>

Closes #4470 from pitrou/ARROW-5334-type-classes-type-suffix and squashes the following commits:

e55a372d9 <Antoine Pitrou> ARROW-5334:  Ensure all type classes end with "Type"