Add dictionary encoded array

[Alex-PLACET]

No description provided.

[JohanMabille]

I think this class is required because of the implementation of bitmap_begin() in the CRTP base class which assumes that each array class has a bitmap attribute:

    template <class D>
    auto array_crtp_base<D>::bitmap_begin() const -> const_bitmap_iterator
    {
        return derived_cast().get_bitmap().begin();
    }

With an alternative implementation not assuming the existence of a bitmap member, but only a way to iterate over a bitmap (either "pysical" or "logical"), such as

    template <class D>
    auto array_crtp_base<D>::bitmap_begin() const -> const_bitmap_iterator
    {
        return derived_cast().bitmap_begin_impl();
    }

where bitmap_begin_impl is defined as return m_bitmap.begin() in the classes with a bitmap attribute, and as return validity_iterator(...) in the dictionary_encoded_layout , we should be able to remove this class and the validity_iterator class, and get something really close to the previous implementation.

This requires defining boilerplate code for bitmap_begin_impl (and bitmap_end_impl) in all array classes but dicitonary_encoded_layout, but that can be done with a macro, and refactored inside an intermediate CRTP later.

[Alex-PLACET]

Addressed

[JohanMabille]

This implemetation is common to all the array classes but the dictionary encoded layout. I think it could be factorize into a free function somewhere (probably in the array_base.hpp file, inside the detail namespace).

[DerThorsten]

there are more layouts with a special bitmap btw.
The run-end-encoded and the sparse and dense union also need a special bitmap

[Alex-PLACET]

Fixed