Implement offsets using new abstraction similarly to View

[mikir]

Currently offsets are implemented by mutable keyword in the Data abstraction. Such solution is simple, however it is not enough for offsets in unions or choices.

Consider the following example:

union TestUnion
{
    uint32 offset;
    string name[];
};

struct TestStruct
{
    TestUnion unionField;
unionField.offset:
    uint32 u32Field;
};

In this case, there is no field offset in generated Data, therefore keyword mutable cannot be used. Because of that, the abstraction View must accept non-const reference to the Data:

template <>
class View<::offsets::offset_in_union::TestUnion>
{
public:
    explicit View(::offsets::offset_in_union::TestUnion& data) noexcept;

    ::zserio::UInt32& offset() const;
    ArrayView<const ::zserio::String> name() const;

    ::offsets::offset_in_union::TestUnion::Tag zserioChoiceTag() const;
    const ::offsets::offset_in_union::TestUnion& zserioData() const;

protected:
    View(::offsets::offset_in_union::TestUnion& data, const View& other) noexcept;

private:
    ::offsets::offset_in_union::TestUnion* m_data;
};

Therefore, it would be better to generate new dedicated abstraction OffsetInitializer similarly to View. This new abstraction could accept non-const reference to the Data and View could accept const reference as expected.