Callback argument wrapping

[HolyBlackCat]

Need to wrap C-style callbacks in something like std::function_ref.

• Convert ...Id arguments to b2::...Ref classes.
• Are there any persistent callbacks that can dangle?
• Are there any useful callbacks outside of classes that need wrapping?

[HolyBlackCat]

Mostly done, now need:

• Add to DynamicTree
• Create a separate system for PreSolve (mem function ptr as template parameter, plus lvalue ref to object?)