Initial Support for Callbacks

[ameily]

Add some initial support for lifting callback functions, which will entail:

• Lift the actual callback function and make sure it is not removed by DCE
• Update function calls that register the callback, for example:

  // original code
  atexit(&my_callback);
  // lifted code
  atexit(&lifted_my_callback);

This initial support will target atexit with the goal of having it extensible for other callbacks.

[ameily]

I pushed my work in progress branch, which is currently broken, to support-callbacks. There are two new passes:

• globalize_function_addresses - identifies potential funciton pointer values and makes them global variables named Callback_<address>
• replace_local_function_pointers - for each global callback address variable, find uses, and replace the use with the lifted function's address

The lifting is crashing for a sample that uses atexit which I believe is because the cast from function to i32 is incorrect.