Delete VariantGetInternalPtr and VariantImplicitConvert, replace with VariantInternalAccessor

[Ivorforce]

Includes / supersedes #105246.
I recommend looking briefly at the above PR, but merging this one (without merging the above first), because it ultimately creates the better end picture.

Explanation

VariantGetInternalPtr, VariantImplicitConvert (introduced temporarily in #105246) and VariantInternalAccessor do pretty much the same thing: They allow users to get values and assign values when working with Variant, when the underlying type is known to match.

Having 3 types for the same task is not a great idea, because code will diverge. In this PR, I am deduplicating functionality by deleting VariantGetInternalPtr and VariantImplicitConvert, and merely using VariantInternalAccessor in all cases.
Doing this allows us to confidently maintain VariantInternalAccessor going forwards, instead of having to maintain a convoluted mess of different ways to do the same thing.

The PR is big and scary, but almost all of the changes are pretty safe search-and-replace.

Notes

Through search-and-replace, some VariantInternalAccessor uses ended up using the following style:

VariantInternalAccessor<A>::get(variant) = b;
// Instead of 
VariantInternalAccessor<A>::set(variant, b);

This looks a bit weird, but it's not detrimental to performance, using a reference to assign to the value. This can get cleaned up if it's confusing in the future.