Add variant_internal.hpp.

[Ivorforce]

This module contains VariantInternalType, VariantInternal, VariantGetInternalPtr, VariantInternalAccessor and VariantDefaultInitializer, facilitating access and manipulation of a Variant's internal value in various ways.

The reference godot variant_internal.hpp is here. I chose a different, less boilerplate-y approach to implementation. It should work the same though. Doing it this way should decrease maintenance (and implementation) effort, since most of these are just copy-pasted code without any type specific logic (with the exception of Object *).

There are a few small differences:

• VariantInternalType does not exist in godot's implementation. I added it because it makes implementation of the other functions easier.
• get_object returns Object * const * in my implementation, but const Object ** in godot's. Technically speaking, godot's implementation is wrong, because it allows manipulation of the contained Object *, even though a const Variant was supplied. Godot uses a C-style cast that acts as a const_cast to avoid cast issues.
• VariantInternalAccessor is not implemented for Object *, even though it is in Godot. I believe it is not possible to replicate the intended behavior without access to Variant::object_assign (or internal ObjData). I made it fail statically using SFINAE.

I think the rest should be the same.

PR contains a unit test for one of the internal accessors.

Depends on #1654.