Rework ExecutionEngine::invoke() to make it more friendly to use from C++

This new invoke will pack a list of argument before calling the
`invokePacked` method. It accepts returned value as output argument
wrapped in `ExecutionEngine::Result<T>`, and delegate the packing of
arguments to a trait to allow for customization for some types.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D95961