SBuffer needs a const version

[guruofquality]

This is the only way we can truly have const correctness with sbuffer. Otherwise, the intrusive pointer just scoots around and gives us the non-const pointer -- the wrong thing is const'd.

This would help with const correctness when getting direct access to SBuffer as an input.

The code will probably just be a typedef of

intrusive_ptr<const SBufferSomething>

I think...