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ARROW-1449: Implement Decimal using only Int128

Author: Phillip Cloud <cpcloud@gmail.com>

Closes #1028 from cpcloud/decimal-single-representation and squashes the following commits:

2ee85a1e [Phillip Cloud] Formatting
0dfceff5 [Phillip Cloud] Don't assign a new string if not necessary
25853e94 [Phillip Cloud] Use STL instead of while loop
bd5e6838 [Phillip Cloud] Fix appveyor warning from stati_cast
04368e56 [Phillip Cloud] Move template implementation to header
18c59489 [Phillip Cloud] Static cast result of std::isdigit to bool
6e967757 [Phillip Cloud] Fix macros
f72f5466 [Phillip Cloud] Formatting
759247e4 [Phillip Cloud] Remove unused header
41fc9a0f [Phillip Cloud] IWYU
ccac0bda [Phillip Cloud] Fix complaint about bool -> Int128
9908d9c3 [Phillip Cloud] Formatting
3d8e399e [Phillip Cloud] Ignore short and long usage linting in decimal-test.cc
8cfa1785 [Phillip Cloud] Make sure we can construct from any integral type
cd7189fa [Phillip Cloud] ARROW-1449: Implement Decimal using only Int128