Simplify callback code using callback_body! macro

[davidhewitt]

While finishing off #797 I had an idea how to remove the run_callback function and instead use a macro. This had three benefits:

1. Now run_callback no longer appears in the stack trace when panicking
2. The new macro is also able to set up the pool with GILPool::new(), pool.python()
3. The new macro is able to automatically convert the output with callback::convert()

As a result, all callback code now uses this macro instead of run_callback, making all callback code shorter and easier to read 😄

[kngwyu]

💯

[kngwyu]

Thank you!