Optimize memory allocations for strings with default values

[mikir]

Consider the following schema:

struct AllocationStruct
{
    string         stringField;
    string         defaultStringField = "Structure Default String Field Must Be Longer Than 32 Bytes";
};

Then, the generated data constructor will allocate memory for the default string value:

AllocationStruct::AllocationStruct(const AllocatorType& allocator) noexcept :
        stringField(allocator),
        defaultStringField(::std::string_view{"Structure Default String Field Must Be Longer Than 32 Bytes"}, allocator)
{}

If such constructed data are used as an input for reading (zserio::read), then the defaultStringField is reallocated and initialized with the new read value. This introduces undesired heap memory fragmantation during reading.

This problem can be probably solved by a new constructor which won't fill default values. However, such constructor can be misused.

In all cases, heap memory fragmantation can be considered as a problem, so we should pay attention to it.