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ARROW-462: [C++] Implement in-memory conversions between non-nested primitive types and DictionaryArray equivalent

Simple usage:

```
Array primitive_array;
DictionaryBuilder<Type> builder;
RETURN_NOT_OK(builder.AppendArray(primitive_array));
std::shared_ptr<Array> dict_encoded_array;
RETURN_NOT_OK(builder.Finish(&dict_encoded_array));
```

Author: Wes McKinney <wes.mckinney@twosigma.com>
Author: Uwe L. Korn <uwelk@xhochy.com>

Closes #812 from xhochy/ARROW-462 and squashes the following commits:

9e289b6a [Wes McKinney] Add a couple doxygen strings. Place WrappedBinary, DictionaryScalar in internal namespace. Add BinaryDictionaryBuilder convenience
4147bdbe [Uwe L. Korn] ARROW-462: [C++] Implement in-memory conversions between non-nested primitive types and DictionaryArray equivalent