* Run hooks for certain predefined object-like macros
These are the "dynamic" macros with their own special execution path.
Most predefined macros can be computed at startup time; those were
already covered by the normal expansion code.
* __VA_OPT__ is supported as a function-like macro
It is referred to as such in the proposal. __VA_ARGS__ is not supported, under the theory that it represents a parameter or set of parameters, and no hooks are called for parameter substitutions.
* Introduce support for C++20 preprocessor features
- add __VA_OPT__ feature to variadic macros
- allow supplying 0 variadic arguments in more cases
- add related unit tests
