Adds safe bindings for set_new_segment_callback

[WindowsCmd]

As the title implies. Introduces set_segment_callback_safe.

[garyhai]

                self.fp.new_segment_callback_user_data = closure as *mut c_void;
                self.fp.new_segment_callback = Some(trampoline::<SegmentCallbackFn>);
                self.segment_calllback_safe = None; 

It is bug here. Should be:

                self.segment_calllback_safe = Some(Box::new(closure));

[thewh1teagle]

As the title implies. Introduces set_segment_callback_safe.

Thanks!
I already use it in the project vibe
and it works great as is (although I created new one lossy to ignore bad utf-8 errors)

Does it gonna be merged?

[tazz4843]

Superseded by #153