DEV-43622 : API change (no functionality change) to fix a design error in LLSD

I made it about a year and a half ago; Zero found it while reading code. I had added a return value to LLSD::insert(), but a) did it wrong, and b) broke the STL-like semantics of insert(). So I've put insert() back to returning void and created LLSD::with(), which does what my earlier insert() did. The compiler then caught all the cases where insert()'s return value were being used, and I changed those to use with() instead.