Change unwinding line arguments from usize to u32

[rprichard]

The file_line arguments to std::rt::unwind::begin_unwind and std::rt::unwind::begin_unwind_fmt takes a &(&str, usize). We changed line! to u32, so we should also update these two functions to also take a u32.

I had noticed this a while ago, but had been putting off fixing them, because it didn't seem time-sensitive. However, I realized today that these functions are stable, so it's technically a breaking-change.

[rprichard]

Also, given the existence of #[allow_internal_unstable], do we really intend these functions to be stable?

[alexcrichton]

Oh yay we definitely don't need to! I've opened #24212 on this topic.

[steveklabnik]

Given that said PR was merged, should this be closed, or not? It's purely internal now, yeah?

[rprichard]

Yes, it's purely internal now, so this issue can be closed.