Add to apply_test.cpp a collect() function that incrementally accumulates an arbitrary number of arguments into a std::vector<std::string>. Construct a std::array<std::string> to pass it, using VAPPLY(). Clarify in header comments that LL::apply() can't call a variadic function with arguments of dynamic size: std::vector or LLSD. The compiler can deduce how many arguments to pass to a function with a fixed argument list; it can deduce how many arguments to pass to a variadic function with a fixed number of arguments. But it can't compile a call to a variadic function with an arguments data structure whose size can vary at runtime. (cherry picked from commit ceed33396266b123896f7cfb9b90abdf240e1eec)