Add callback and use IRAM heap if configured

[dkerr64]

Offering enhancements we added when developing HomeKit-RATGDO project...

1. Add an optional callback to user code after crash is saved.
2. Use IRAM heap if compiled with shared 2nd heap option. See documentation and here. This saves valuable space for the application in the main heap.
3. Fix output of certain exception types so can be properly decoded