This macro block encapsulates the common pattern of switching to the prior (formerly called old) mem context to return results from a function.
Also rename MEM_CONTEXT_OLD() to memContextPrior(). This violates our convention of macros being in all caps but memContextPrior() will become a function very soon so this will reduce churn.
Most of the *Free() functions are pretty generic so add macros to make creating them as easy as possible.
Create a distinction between *Free() functions that the caller uses to free memory and callbacks that free third-party resources. There are a number of cases where a driver needs to free resources but does not need a normal *Free() because it is handled by the interface.
Add common/object.h for macros that make object maintenance easier. This pattern can also be used for many more object functions.