Move to stable rustc

[rinon]

We are currently using the following nightly-only features:

Unsafe pointer manipulation:

• [ ] #![feature(const_raw_ptr_to_usize_cast)]
• [ ] #![feature(const_transmute)]
• [ ] #![feature(ptr_wrapping_offset_from)]

C ABI:

• [x] #![feature(extern_types)]

Convenience, can be removed:

• [x] #![feature(label_break_value)]
• [ ] #![feature(main)]

[rinon]

label_break_value removed in 78f0686