`DAP_IMPLEMENT_STRUCT_TYPEINFO_EXT` and `DAP_STRUCT_TYPEINFO_EXT` are two new flavors of `DAP_IMPLEMENT_STRUCT_TYPEINFO` and `DAP_STRUCT_TYPEINFO` that allow you to derive message types.
This involved a bit of reworking on the serializer interfaces.
Added test.
Issue: #32
Empty structs were being serialized as `null`, when they should have been serialized as `{}`.
This was due to the type inference on the serializer - where no calls to `field()` would result in the default `null` type.
To solve this, the `serialize(const void* object, const std::initializer_list<Field>&)` inline helper has been promoted to a virtual function (and renamed to `fields()`).
The JSON serializer implementation of this now first sets the object type to `object`, even if there are no fields to serialize.
Added test for this.
Fixes: #10
