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ARROW-3321: [C++] Improve integer parsing performance

Before:
```
---------------------------------------------------------------------
Benchmark                              Time           CPU Iterations
---------------------------------------------------------------------
BM_IntegerParsing<Int8Type>         1388 ns       1387 ns     502861   5.49911M items/s
BM_IntegerParsing<Int16Type>        1475 ns       1475 ns     468724   5.17179M items/s
BM_IntegerParsing<Int32Type>        1730 ns       1729 ns     405693   4.41194M items/s
BM_IntegerParsing<Int64Type>        2131 ns       2131 ns     328192   3.58034M items/s
BM_IntegerParsing<UInt8Type>        1238 ns       1238 ns     572573   6.16483M items/s
BM_IntegerParsing<UInt16Type>       1302 ns       1301 ns     537960   5.86206M items/s
BM_IntegerParsing<UInt32Type>       1391 ns       1391 ns     502859    5.4857M items/s
BM_IntegerParsing<UInt64Type>       1637 ns       1637 ns     427832     4.661M items/s
BM_FloatParsing<FloatType>          4437 ns       4436 ns     156887   1.71973M items/s
BM_FloatParsing<DoubleType>         4593 ns       4592 ns     152459   1.66129M items/s
```

After:
```
---------------------------------------------------------------------
Benchmark                              Time           CPU Iterations
---------------------------------------------------------------------
BM_IntegerParsing<Int8Type>           23 ns         23 ns   29800687   324.788M items/s
BM_IntegerParsing<Int16Type>          27 ns         27 ns   26593165   287.438M items/s
BM_IntegerParsing<Int32Type>          34 ns         34 ns   20689813   226.211M items/s
BM_IntegerParsing<Int64Type>          49 ns         49 ns   14256379   155.424M items/s
BM_IntegerParsing<UInt8Type>          17 ns         17 ns   42295211   454.911M items/s
BM_IntegerParsing<UInt16Type>         16 ns         16 ns   42663172   464.397M items/s
BM_IntegerParsing<UInt32Type>         21 ns         21 ns   33372432   363.209M items/s
BM_IntegerParsing<UInt64Type>         33 ns         33 ns   21502295   234.255M items/s
BM_FloatParsing<FloatType>          4554 ns       4553 ns     153207   1.67565M items/s
BM_FloatParsing<DoubleType>         4579 ns       4578 ns     152304   1.66651M items/s
```

Author: Antoine Pitrou <antoine@python.org>

Closes #2619 from pitrou/ARROW-3321-faster-int-conversion and squashes the following commits:

9834b28e6 <Antoine Pitrou> ARROW-3321:  Improve integer parsing performance