Add option to override behavior of memory functions

[wjwwood]

Currently you can only get a callback to notify you when a memory function is called, but you have no ability access the arguments to the memory function called nor can you influence whether or not the original memory functions are called or redirect to a different allocator.

A use case for this might be to fail on a specific call to malloc in a test or to completely replace the system allocators in a deployed program.