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

[xhochy]

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));

[wesm]

You could use PrimitiveArray here and cast the raw_data() to const Scalar*

[wesm]

I don't know whether overloaded methods or differently-named methods is better.

As an aside, should start writing more Doxygen docstrings for new APIs (and making a pass over our existing APIs). Please remind me to write more documentation in my PRs, too =)

[xhochy]

Overloaded methods did sadly not work in combination with the templates. Thus I renamed them to avoid the compiler errors.

[wesm]

Looks like the tests only failed due to anaconda.org problems

[wesm]

+1, will await build. I made a couple small tweaks